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.

Closes #6505