Make ReadWriteAbility::_canAcceptNonLocalWrites AtomicWord<bool> to prevent torn reads.

Currently, we are accessing _canAcceptNonLocalWrites in an unsafe manner by NOT taking any RSTL lock.  This can lead to torn read/ undefined behavior if concurrently a step up/step down thread is modifying the value.

• service_entry_point_common.cpp - execCommandDatabase
• line 667.
• line 691.

• Command_can_run_here.cpp - commandCanRunHere. And, below are the callers.
• Explain cmd.
• execCommandDatabase.

And, they all are best effort checks. So, it's ok to just make reading _canAcceptNonLocalWrites thread safe.