MXS-1886 Better auto-rejoin error description and tolerance

Auto-rejoin now explains more accurately if a server cannot be joined due
to conflicting gtid.

Also, auto-rejoin is no longer disabled if a join fails. Usually the fail
is due to the server not replying fast enough with query completion. The
query is often completed anyways. This can lead to some log spam.

server/modules/monitor/mysqlmon.h

@@ -76,6 +76,7 @@ typedef struct
     int external_master_port;      /**< External master port */
     bool auto_rejoin;              /**< Attempt to start slave replication on standalone servers or servers
                                         replicating from the wrong master. */
+    bool warn_cannot_rejoin;       /**< Print warning if auto_rejoin fails because of invalid gtid:s? */
     bool enforce_read_only_slaves; /**< Should the monitor set read-only=1 on any slave servers. */
     int n_excluded;                /**< Number of excluded servers */
     MXS_MONITORED_SERVER** excluded_servers; /**< Servers banned for master promotion during auto-failover. */