Allow direct access when using a registered process

Possible scenarios came up for Cloudant FogBugz case 25904 which saw a large message queue on ddoc_cache_opener which uses ets_lru. This may or may not directly address this problem but the patch is simple and should provide some small gains for the ddoc_cache when opening documents w/o specific revisions.

The match_object API currently takes both atoms and pids. This change allows the atom case to skip a synchronous call to the gen_server state. The old path remains to allow pids to continue to work through the old call path since it can't easily infer the lru objects table name.