Add a test framework for recovery

Add a test framework for recovery

This long-awaited framework is an expansion of the existing PostgresNode
stuff to support additional features for recovery testing; the recovery
tests included in this commit are a starting point that cover some of
the recovery features we have.  More scripts are expected to be added
later.

Author: Michaël Paquier, a bit of help from Amir Rohan
Reviewed by: Amir Rohan, Stas Kelvich, Kyotaro Horiguchi, Victor Wagner,
Craig Ringer, Álvaro Herrera
Discussion: http://www.postgresql.org/message-id/CAB7nPqTf7V6rswrFa=q_rrWeETUWagP=h8LX8XAov2Jcxw0DRg@mail.gmail.com
Discussion: http://www.postgresql.org/message-id/trinity-b4a8035d-59af-4c42-a37e-258f0f28e44a-1443795007012@3capp-mailcom-lxa08