handle defs from findCache no longer existing. (#1287)
handle defs from findCache no longer existing. (#1287)

- Also when deleting series, invalidate the findCache after
  releasing the write lock.  This reduces lock time.  The findCache
  invalidates entries asynchronously, so there is no need to queue
  the invalidations while holding a lock.
replay/metrictank,replay/metrictank,raintank/metrictank,raintank/metrictank,raintank/metrictank,raintank/raintank-metric,raintank/metrictank,raintank/raintank-metric,replay/metrictank,raintank/raintank-metric,replay/metrictank