Cache ObjectContexts across OSDOps

ObjectContext is constructed when op begins and destroyed when op finished, it costs about 100us each op.

This patch will cache the ObjectContext in a RandomCache to keep it in memory, and trim the cache when its correspondent object is removed or the cache count reach the osd_object_context_caches_count whose default is 4096.

This patch will reduce nearly 100us for each IO if hit cache.

Signed-off-by: Dong Yuan <yuandong1222@gmail.com>
Add a performance counter to track hit and miss rates.