Introducing a new persistent connection caching system using "bundles".

A bundle is a list of all persistent connections to the same host.
The connection cache consists of a hash of bundles, with the
hostname as the key.
The benefits may not be obvious, but they are two:

1) Faster search for connections to reuse, since the hash
   lookup only finds connections to the host in question.
2) It lays out the groundworks for an upcoming patch,
   which will introduce multiple HTTP pipelines.

This patch also removes the awkward list of "closure handles",
which were needed to send QUIT commands to the FTP server
when closing a connection.
Now we allocate a separate closure handle and use that
one to close all connections.

This has been tested in a live system for a few weeks, and of
course passes the test suite.

lib/urldata.h

@@ -795,8 +795,8 @@ struct connectdata {
                  consideration (== only for pipelining). */
 
   /**** Fields set when inited and not modified again */
-  long connectindex; /* what index in the connection cache connects index this
-                        particular struct has */
+  long connection_id; /* Contains a unique number to make it easier to
+                         track the connections in the log output */
 
   /* 'dns_entry' is the particular host we use. This points to an entry in the
      DNS cache and it will not get pruned while locked. It gets unlocked in
@@ -924,7 +924,6 @@ struct connectdata {
                               handle */
   bool server_supports_pipelining; /* TRUE if server supports pipelining,
                                       set after first response */
-
   struct curl_llist *send_pipe; /* List of handles waiting to
                                    send on this pipeline */
   struct curl_llist *recv_pipe; /* List of handles waiting to read
@@ -934,7 +933,6 @@ struct connectdata {
   struct curl_llist *done_pipe; /* Handles that are finished, but
                                    still reference this connectdata */
 #define MAX_PIPELINE_LENGTH 5
-
   char* master_buffer; /* The master buffer allocated on-demand;
                           used for pipelining. */
   size_t read_pos; /* Current read position in the master buffer */
@@ -1011,6 +1009,8 @@ struct connectdata {
     TUNNEL_CONNECT, /* CONNECT has been sent off */
     TUNNEL_COMPLETE /* CONNECT response received completely */
   } tunnel_state[2]; /* two separate ones to allow FTP */
+
+   struct connectbundle *bundle; /* The bundle we are member of */
 };
 
 /* The end of connectdata. */
@@ -1146,18 +1146,6 @@ struct auth {
                    be RFC compliant */
 };
 
-struct conncache {
-  /* 'connects' will be an allocated array with pointers. If the pointer is
-     set, it holds an allocated connection. */
-  struct connectdata **connects;
-  long num;           /* number of entries of the 'connects' array */
-  enum {
-    CONNCACHE_PRIVATE, /* used for an easy handle alone */
-    CONNCACHE_MULTI    /* shared within a multi handle */
-  } type;
-};
-
-
 struct UrlState {
   enum {
     Curl_if_none,
@@ -1165,13 +1153,20 @@ struct UrlState {
     Curl_if_multi
   } used_interface;
 
-  struct conncache *connc; /* points to the connection cache this handle
-                              uses */
+  /* Points to the connection cache */
+  struct conncache *conn_cache;
 
   /* buffers to store authentication data in, as parsed from input options */
   struct timeval keeps_speed; /* for the progress meter really */
 
-  long lastconnect;  /* index of most recent connect or -1 if undefined */
+  struct connectdata *pending_conn; /* This points to the connection we want
+                                       to open when we are waiting in the
+                                       CONNECT_PEND state in the multi
+                                       interface. This to avoid recreating it
+                                       when we enter the CONNECT state again.
+                                    */
+
+  struct connectdata *lastconnect; /* The last connection, NULL if undefined */
 
   char *headerbuff; /* allocated buffer to store headers in */
   size_t headersize;   /* size of the allocation */
@@ -1250,14 +1245,6 @@ struct UrlState {
   /* for FTP downloads: how many CRLFs did we converted to LFs? */
   curl_off_t crlf_conversions;
 #endif
-  /* If set to non-NULL, there's a connection in a shared connection cache
-     that uses this handle so we can't kill this SessionHandle just yet but
-     must keep it around and add it to the list of handles to kill once all
-     its connections are gone */
-  void *shared_conn;
-  bool closed; /* set to TRUE when curl_easy_cleanup() has been called on this
-                  handle, but it is kept around as mentioned for
-                  shared_conn */
   char *pathbuffer;/* allocated buffer to store the URL's path part in */
   char *path;      /* path to use, points to somewhere within the pathbuffer
                       area */
@@ -1593,6 +1580,8 @@ struct UserDefined {
   bool tcp_keepalive;    /* use TCP keepalives */
   long tcp_keepidle;     /* seconds in idle before sending keepalive probe */
   long tcp_keepintvl;    /* seconds between TCP keepalive probes */
+
+  size_t maxconnects;  /* Max idle connections in the connection cache */
 };
 
 struct Names {