Add a new piece of data to NoteDbChangeState indicating that a change
is read-only until a specified future time. We use a timestamp
instead of a separate state to support a migration step where the job
doing the migration might die, so the change automatically becomes
read-write again without manual intervention.
This read-only state only needs to be checked while updating
ReviewDb, which includes an atomic update to the noteDbState field.
There are exactly two locations that do this atomic update:
ChangeRebuilderImpl#execute and BatchUpdate.ChangeTask#call.
We don't bother with attempting to double-check this field when doing
the corresponding NoteDb write, for example from NoteDbUpdateManager.
It's not possible when we're about to write to NoteDb to do a
race-free atomic read of both the read-only state from ReviewDb and
the ref in NoteDb that we're about to update. So, we just let the
NoteDb write attempt proceed. This turns out to not affect the
correctness of the migration process; the race condition is explained
in more detail in the commit that implements the migration process.
Change-Id: If21353582f678b8788285bc2e9f7b50a5c14f6d4
a96742c035