Check if a secondary is connected before demoting primary

[ioguix]

In sub pgsql_demote, we need to make sure that at least one secondary is connected when demoting the primary on a switchover scenario.

If Pacemaker move the master role to another node and demote the old one as a slave, we should raise an error ("soft" failure ?) if the new master-to-be is not up-to-date with the current master.

[ioguix]

This has been fixed in 2.0 in commit 952cb3c

The solution adopted is to check if the slave received the shutdown checkpoint from the master after it has been demoted.