Replace wsrep_on() with trx_t::is_wsrep() where possible.

Also, rename some functions to member functions and
remove unused DBUG_EXECUTE_IF instrumentation:

trx_t::commit(): Renamed from trx_commit().

trx_t::commit_low(): Renamed from trx_commit_low().

trx_t::commit_in_memory(): Renamed from trx_commit_in_memory().