First cut of primary server checking.

First cut of primary server checking.

For now followings are implemented:

- Check all backend nodes starting node 0.

- If primary nodes appear twice or more, the second one or after are
  assumed invalid.

- Such invalid node will be degenerated at the next convenient
  time. Currently such timing is at the start up of Pgpool-II. This is
  apparently insufficient and should be improved later.

TODO:
- Verify primary nodes using pg_stat_wal_receiver.

- More chances to verify node status. Maybe in the same timing as
  streaming replication delay checking?

- Add new GUCs to control of this feature.