PROBLEM:
i_innodb.innodb_bug14712710 fails sporadically on PB2 branches
with warning debug sync point timed out and error code of
ER_DEBUG_SYNC_TIMEOUT. The problem could be that the wait
thread which is  waiting at a sync point for a signal from the
signalling thread could not process the signal because another
signalling could have overwritten the debug_sync_global.ds_signal.
This causes the waiting thread to wait till timeout expires leading
to ER_DEBUG_SYNC_TIMEOUT error. This happens because the wait thread
when woken up from a signal could contend with another signal for
grabbing the debug_sync_global.ds_mutex. If the signal thread succeeds
in the mutex, then new signal will be overwritten in debug_sync_global.
ds_signal before the wait thread could process it. Also there exists
the possibly of wait thread arriving late at a wait point before some
signal threads could have overwritten the signal that the wait thread
possibly wants to wait for. NOTE: The mysql documentation also needs 
to be modified.
FIX:
In debug_sync_global maintain list of signals that have been signalled
in a std::set and remove the signal from the list when the corressponding
wait thread has processed that signal. Introduce a wait_for attribute
no_clear_event that allows signal not to be cleared from the global thus
allowing a single signal event to wake up multiple threads that wait for
this signal. This is useful for the test cases rpl.rpl_semisync_deadlock
and i_rpl.rpl_lost_events_on_rotate.