Optimize database decref by avoiding locking with refs > 1

Optimize database decref by avoiding locking with refs > 1

Previously, this function always acquires a node write lock if it
might need node cleanup in case the reference decrements to 0.  In
fact, the lock is unnecessary if the reference is larger than 1 and it
can be optimized as an "easy" case. This optimization could even be
"necessary". In some extreme cases, many worker threads could repeat
acquring and releasing the reference on the same node, resulting in
severe lock contention for nothing (as the ref wouldn't decrement to 0
in most cases). This change would prevent noticeable performance
drop like query timeout for such cases.

Co-authored-by: JINMEI Tatuya <jtatuya@infoblox.com>
Co-authored-by: Ondřej Surý <ondrej@isc.org>
(cherry picked from commit 7f4471594db157b61218ee6377e6ba0887092a41)