[SCHEMA] Add is_draft column to Message

This is part of the Inbox datastore API that we're going to expose.
squash! tmp doc updates
Document ImapAccount.sync_lock()
Add a write lock for ImapAccount.

We need this in order to be able to have different processes
updating data in the datastore, e.g. API functions from Inbox
clients and the mail sync engine.
Switch IMAP mail sync to use account-wide write lock.

We can't just use an in-memory lock anymore because we're going to have
other processes writing to the database too.
Wrap mailsync message update/delete in syncmanager_lock too.

These can end up modifying datastore data too (i.e. tables other than ImapUid).
Properly fill out Message.is_draft.

(This hadn't been implemented yet.)
Remove totally redundant comment.