Quorum commit for multiple synchronous replication.

Hi all,

In 9.6 development cycle, we had been discussed about configuration
syntax for a long time while considering expanding.
As a result, we had new dedicated language for multiple synchronous
replication, but it supports only priority method.
We know that quorum commit is very useful for many users and can
expand dedicated language easily for quorum commit.
So I'd like to propose quorum commit for multiple synchronous replication here.

The followings are changes attached patches made.
- Add new syntax 'Any N ( node1, node2, ... )' to
synchornous_standby_names for quorum commit.
- In quorum commit, the master can return commit to client after
received ACK from *at least* any N servers of listed standbys.
- sync_priority of all listed servers are same, 1.
- Add regression test for quorum commit.

I was thinking that the syntax for quorum method would use '[ ... ]'
but it will be confused with '( ... )' priority method used.
001 patch adds 'Any N ( ... )' style syntax but I know that we still
might need to discuss about better syntax, discussion is very welcome.
Attached draft patch, please give me feedback.

Regards,

--
Masahiko Sawada