Explicit support for 2-phase commits

[tdz]

At FrOSCon 2017, someone asked about support for 2-phase commits. Currently this can be done by running the first phase in the module's apply() callbacks and the second phase in the finish() callback.

It might make sense to build explicit support for 2pc instead.

[tdz]

Here's an idea for an implementation:

1. Events in the transaction log could contain a field noting their phase.
2. Appending an event would allow setting the field.
3. During apply, the transaction manager walks over the log once for each required phase.
4. A set of database/network/etc modules could apply changes to structured data during the first phase; with a common record number handed out by a record-manager module
5. The record-manager module commits the transaction's record number as part of the second phase; thus making the record active.