Re: Support for N synchronous standby servers - take 2

Hi,

Attached patch is a rough patch which supports multi sync replication
by another approach I sent before.

The new GUC parameters are:
* synchronous_standby_num, which specifies the number of standby
servers using sync rep. (default is 0)
* synchronous_replication_method, which specifies replication method;
priority or quorum. (default is priority)

The behaviour of 'priority' and 'quorum' are same as what we've been discussing.
But I write overview of these here again here.

[Priority Method]
The standby server has each different priority, and the active standby
servers having the top N priroity are become sync standby server.
If synchronous_standby_names = '*', the all active standby server
would be sync standby server.
If you want to set up standby like 9.5 or before, you can set
synchronous_standby_num = 1.