Analysis:
========
At runtime, some GTID_SET objects are instrumented with a
performance schema mutex key of 0, which is incorrect (not
instrumented).

The root cause is the declaration of constructors, like:

Gtid_set(Sid_map *sid_map, Checkable_rwlock *sid_lock= NULL
#ifdef HAVE_PSI_INTERFACE
        ,PSI_mutex_key free_intervals_mutex_key= 0
#endif
        );

The issue is giving a default value of 0 when the mutex key
is not passed. This allows some caller to not pass any key,
causing a bug.


Fix:
===
Moved the PSI_mutex_key from Gtid_state class to Gtid_set
class.