Fixing a post push pb2 valgrind failure:
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Analysis: In commitTrx(), when a thread is waiting for the signal
on a condition variable (entry->cond) after releasing LOCK_binlog_,
it can happen that this 'entry' node in active transaction list
deleted by another parallel thread which is doing semi sync off
(::disableMaster or ::switch_off or resetMaster). This leads to
invalid memory read issue.

Fix: Now with the new design, only the thread who is doing commit
is responsible for deleting the active transaction node. All other
operations (disableMaster, switch_off and resetMaster) will not
delete nodes from the active transaction list. If disableMaster
and resetMaster finds that there are no nodes in the list,
it will delete active_tranxs_ node.