watchable store: extract func that gets next log seq

Extracting the function that returns the next sequence number to be
used for a new commit. The function is optimized with a binary search.
The same function will be used to implement the GetResumeMarker().

Change-Id: I89fd5899f82e0e25e666873ee8a79a1d095e82bb
4 files changed
tree: 5e43d5249cd63cfcb9ebfa0d4b92bc3e9f560839
  1. services/
  2. syncbase/