Add an ability to set the SSL read buffer size

This capability is required for read pipelining. We will only read in as
many records as will fit in the read buffer (and the network can provide
in one go). The bigger the buffer the more records we can process in
parallel.

Reviewed-by: Tim Hudson <tjh@openssl.org>

ssl/ssl_locl.h

@@ -843,6 +843,9 @@ struct ssl_ctx_st {
     /* Up to how many pipelines should we use? If 0 then 1 is assumed */
     unsigned int max_pipelines;
 
+    /* The default read buffer length to use (0 means not set) */
+    size_t default_read_buf_len;
+
 #  ifndef OPENSSL_NO_ENGINE
     /*
      * Engine to pass requests for client certs to