Support synchronous replication

[Winslett]

Asynchronous replication does not work for everyone. Add ability to use synchronous.

Includes:

• significant documentation on making proper replication choices for your business requirements
• reordered the initialization process. I could not create a user for replication on a database in synchronous mode. The database would stall while confirming the CREATE USER write against a follower database, which did not exist yet. Thus, on initialization of the first member, Postgres is started in a local-only mode to configure replication users. Then Postgres is stopped, and started with the configuration set by the YAML file.

[titanous]

Check out Joyent's Manatee state machine for an example of how to do an HA Postgres system that is safe. It actually requires a ton of work (and the use of sync replication) to eliminate all of the data-mangling failure cases.