RedisBagOStuff: if no alternatives, skip master link status check

RedisBagOStuff: if no alternatives, skip master link status check

If RedisBagOStuff::getConnection() is able to establish a connection, only
check the master link status if automatic failover is enabled and if there are
other viable servers left to consider. If there are no servers left to
consider, or if automatic failover is not configured, just return the
connection handle without subjecting it to further tests.

This will have the side-effect of making RedisBagOStuff compatible with
Nutcracker, which does not implement the INFO command. This is because when
MediaWiki is configured to use Nutcracker, the server pool will consist of a
single server (namely, Nutcracker itself), and thus there will be no other
server to consider, so INFO will never be executed.

Change-Id: I3812ec5a0b22df122bdf44350bc0496574c02ce8