Improved logging when a join request is not executed because local node is no longer master
Added ServiceDisruptionScheme(s) and testAckedIndexing

This commit adds the notion of ServiceDisruptionScheme allowing for introducing disruptions in our test cluster. This
abstraction as used in a couple of wrappers around the functionality offered by MockTransportService to simulate various
network partions. There is also one implementation for causing a node to be slow in processing cluster state updates.

This new mechnaism is integrated into existing tests DiscoveryWithNetworkFailuresTests.

A new test called testAckedIndexing is added to verify retrieval of documents whose indexing was acked during various disruptions.