indexer: async stats calculation in timekeeper
indexer: async stats calculation in timekeeper

stats calculation calls KV to latest seqno.
this shouldn't block timekeeper main loop.

Change-Id: Ibc2981a4d1516eebf47ad0b983067aea9fd5ab2d

jchris/indexing,jchris/indexing,jchris/indexing,jchris/indexing