Handle unexpected responses to COM_CHANGE_USER

When an unexpected response to a COM_CHANGE_USER is received, it is now
processes and discarded instead of treated as an error. This will allow
further analysis of the situation in addition to possibly solving some of
the problems that the persistent connections have.

Added extra info level logging to relevant parts of the code that deal
with the COM_CHANGE_USER reply processing. This information should allow
tracking of the response state for debugging purposes.

server/modules/protocol/MySQL/mysql_common.c

@@ -104,6 +104,7 @@ MySQLProtocol* mysql_protocol_init(DCB* dcb, int fd)
     p->protocol_command.scom_nbytes_to_read = 0;
     p->stored_query = NULL;
     p->extra_capabilities = 0;
+    p->ignore_reply = false;
 #if defined(SS_DEBUG)
     p->protocol_chk_top = CHK_NUM_PROTOCOL;
     p->protocol_chk_tail = CHK_NUM_PROTOCOL;