set carbon cache/creates/updates on shutdown

setting an artificial cache limit here was really messing
things up.  Unless we are reserving those resources for
something more important lets give the cache queue
all we've got and explicitely throttle new creations.  cache
will manage itself when their are natural drops by doing no
less favorable of a thing than it was doing in hitting it's
configured ceiling -- spinning it's wheels and dropping tons
of metrics.

these changes took us from dropping 75K metrics with a load 16-20
to handling 125K metrics with a load avg of 4

Change-Id: Ia43426eb713936fa39c8f2c549aa176ee2b0429f