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 partitions. There is also one implementation for causing a node to be slow in processing cluster state updates.

This new mechanism 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.

**NOTE:** this PR is against the feature/improve_zen branch
Maybe this method should be in the TestCluster?