Implement performFailover logic for ObserverReadProxyProvider.

Currently ObserverReadProxyProvider inherits performFailover() method from ConfiguredFailoverProxyProvider, which simply increments the index and switches over to another NameNode. The logic for ORPP should be smart enough to choose another observer, otherwise it can switch to a SBN, where reads are disallowed, or to an ANN, which defeats the purpose of reads from standby.
This was discussed in HDFS-12976.