Use RFC 3986 compliant addresses in log messages

When log messages are written with both address and port information, IPv6
addresses can cause confusion if the normal address:port formatting is
used. The RFC 3986 suggests that all IPv6 addresses are expressed as a
bracket enclosed address optionally followed by the port that is separate
from the address by a colon.

In practice, the "all interfaces" address and port number 3306 can be
written in IPv4 numbers-and-dots notation as 0.0.0.0:3306 and in IPv6
notation as [::]:3306. Using the latter format in log messages keeps the
output consistent with all types of addresses.

The details of the standard can be found at the following addresses:

     https://www.ietf.org/rfc/rfc3986.txt

     https://www.rfc-editor.org/std/std66.txt

server/modules/routing/readwritesplit/rwsplit_mysql.c

@@ -369,7 +369,7 @@ void check_session_command_reply(GWBUF *writebuf, sescmd_cursor_t *scur, backend
 
         ss_dassert(len + 4 == GWBUF_LENGTH(scur->scmd_cur_cmd->my_sescmd_buf));
 
-        MXS_ERROR("Failed to execute session command in %s:%d. Error was: %s %s",
+        MXS_ERROR("Failed to execute session command in [%s]:%d. Error was: %s %s",
                   bref->ref->server->name,
                   bref->ref->server->port, err, replystr);
         MXS_FREE(err);