Previously the SetConsistentIndex() is called during the apply workflow,
but it's outside the db transaction. If a commit happens between SetConsistentIndex
and the following apply workflow, and etcd crashes for whatever reason right
after the commit, then etcd commits an incomplete transaction to db.
Eventually etcd runs into the data inconsistency issue.
In this commit, we move the SetConsistentIndex into a txPostLockHook, so
it will be executed inside the transaction lock.
Thanks to this change:
- all the maps bucket -> buffer are indexed by int's instead of
string. No need to do: byte[] -> string -> hash conversion on each
access.
- buckets are strongly typed in backend/mvcc API.
