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.
Every transaction committed to backend is writing most recent consistent_index.
Makes sure that even automatically trigger commits of batch-transactions
stays "really" consistent a.d. the most recent WAL log index applied.
Motivation:
- ServerConfig is part of 'embed' public API, while etcdserver is more 'internal'
- EtcdServer is already too big and config is pretty wide-spread leaf
if we were to split etcdserver (e.g. into pre & post-apply part).
