Split commits into two phases: Prepare and Apply

[DavidBuchanan314]

"Prepare" is a read-only phase, generating a description of the required changes to be made to the repo.

"Apply" actually persists those changes to the db (making sure no other commits have been applied in the meantime!)

Currently both are rolled into one - the giant apply_writes function in repo_ops.py (nb: the verb "apply" is overloaded here - think of a better one?)

There are several benefits to this change:

1. Improved testability (much easier to isolate the "Prepare" phase for testing).
2. Opens the door for implementing client-signed commits (but this is far from the only change needed for that).
3. Reduced db transaction granularity (two medium-sized transactions vs one big one).

[DavidBuchanan314]

Should probably separate out the core commit logic from the firehose message generation logic, too

[DavidBuchanan314]

re: verbs - "persist" seems like a good word for the second phase