Parallel replication incorrectly caches charset between worker threads

As reported by nanyi607rao on maria-developers@lists.launchpad.net:

> If character_set in different Query_log_events changed, worker threads may
apply them with wrong character_set. the codes leading this problem is in
Query_log_event::do_apply_event, that is:
> if (charset_inited)
> {
> if (rli->cached_charset_compare(charset))

Clearly, the rli cannot be used to cache charset between Queries when parallel
replication is used.

Probably the solution is to move the cached_charset stuff from Relay_log_info
into rpl_group_info.