push_back() is not reentrant with pop_front(), used by the evictor. If
reclaimer runs when std::deque allocates a new node it will get
corrupted. Fix by runnning push_back() under reclaim lock.
In some cases region may be in a state where it is not empty and
nothing could be evicted from it. For example when creating the first
entry, reclaimer may get invoked during creation before it gets
linked. We therefore can't rely on emptiness as a stop condition for
reclamation, the evction function shall signal us if it made forward
progress.
