imap: Changed response tag generation to be completely unique

Updated the automatic response tag generation to follow the examples
given in RC3501, which list a 4 character string such as A001, A002,
etc.

As a unique identifier should be generated for each command the string
generation is based on the connection id and the incrementing command
id.

tests/data/test1321

@@ -56,11 +56,11 @@ imap://%HOSTIP:%IMAPPORT/1321 -u user:secret -p -x %HOSTIP:%PROXYPORT
 ^User-Agent: curl/.*
 </strip>
 <protocol>
-B CAPABILITY
-C LOGIN user secret
-D SELECT 1321
-A FETCH 1 BODY[TEXT]
-B LOGOUT
+A001 CAPABILITY
+A002 LOGIN user secret
+A003 SELECT 1321
+A004 FETCH 1 BODY[TEXT]
+A005 LOGOUT
 </protocol>
 <proxy>
 CONNECT %HOSTIP:%IMAPPORT HTTP/1.1