limit number of old chunks that can be added to the writeQueue.
limit number of old chunks that can be added to the writeQueue.

fixes #84
If we try and write more chunks to the writeQueue then the capacity
of the writeQueue in one go while holding the lock, we can end up
in a deadlock.  This commit ensures that this never happens.

OOM-Killer/metrictank,OOM-Killer/metrictank,raintank/metrictank,raintank/raintank-metric,replay/metrictank,replay/metrictank,raintank/raintank-metric,OOM-Killer/metrictank,replay/metrictank,raintank/metrictank,raintank/metrictank,raintank/metrictank,replay/metrictank,raintank/raintank-metric