Standalone synchronous master

This patch implements the following TODO item:

Add a new "eager" synchronous mode that starts out synchronous but reverts to asynchronous after a failure timeout period
This would require some type of command to be executed to alert administrators of this change.
http://archives.postgresql.org/pgsql-hackers/2011-12/msg01224.php

This patch implementation is in the same line as it was given in the earlier thread.
Some Of the additional important changes are:

1. Have added two GUC variable to take commands from user to be executed

a. Master_to_standalone_cmd: To be executed before master switches to standalone mode.

b. Master_to_sync_cmd: To be executed before master switches from sync mode to standalone mode.

2. Master mode switch will happen only if the corresponding command executed successfully.

3. Taken care of replication timeout to decide whether synchronous standby has gone down. i.e. only after expiry of

wal_sender_timeout, the master will switch from sync mode to standalone mode.

Please provide your opinion or any other expectation out of this patch.

I will add the same to November commitFest.

Thanks and Regards,
Kumar Rajeev Rastogi