Add DBProperty to return number of snapshots and time for oldest snapshot

Summary:
Add a counter in SnapshotList to show number of snapshots. Also a unix timestamp in every snapshot.
Add two DB Properties to return number of snapshots and timestamp of the oldest one.

Test Plan: Add unit test checking

Reviewers: yhchiang, rven, igor

Reviewed By: igor

Subscribers: leveldb, dhruba, MarkCallaghan

Differential Revision: https://reviews.facebook.net/D29919