ENG-6009: Fix race at the end of rejoin/join that may cause MP deadlock.

Since the m_rejoinDataPending flag in RealVoltDB is set asynchronously, sites
could have inconsistent views of what the value is during the execution of a
sysproc at the end of rejoin/join. Created a method to atomically check the flag
and determine if it's safe to execute a sysproc, @SnapshotSave fragment to be
specific. For the same MP sysproc, the method always returns the same decision,
even though the flag could flip in between the calls.