2937. [bug] Worked around an apparent race condition in over

2937.	[bug]		Worked around an apparent race condition in over
			memory conditions.  Without this fix a DNS cache DB or
			ADB could incorrectly stay in an over memory state,
			effectively refusing further caching, which
			subsequently made a BIND 9 caching server unworkable.
			This fix prevents this problem from happening by
			polling the state of the memory context, rather than
			making a copy of the state, which appeared to cause
			a race.  This is a "workaround" in that it doesn't
			solve the possible race per se, but several experiments
			proved this change solves the symptom.  Also, the
			polling overhead hasn't been reported to be an issue.
			This bug should only affect a caching server that
			specifies a finite max-cache-size.  It's also quite
			likely that the bug happens only when enabling threads,
			but it's not confirmed yet. [RT #21818]