Massively speed up SharedMem.hash_stats
Massively speed up SharedMem.hash_stats

Summary: Our slow implementation of `hh_hash_used_slots` was really slowing us down for large repos! We were walking the whole hash table, but we can easily just keep a count.

Reviewed By: jbrown215

Differential Revision: D16939688

fbshipit-source-id: 8e80a02a321664b98668666638222be3729c75fd

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow