Set up a mocked-up objects for testing replica-set causal consistent reads.
The fixture should provide the following functionality:

• set LogicalClock
• write to an opLog
• read from an opLog
• write to secondary and primary nodes (separately by setting repl coordinator mode)
• read from the secondary and primary nodes
• clocksource skew
• disable/enable readConcernMajority

Implementation:

• Use existing LogicalClockTest class can be used as a base, inherit from ShardingMongodTestFixture
• override setUp to
  a) set up logicalClock on serviceContext
  b) add a clockSource mock
  c) add a keyManager mock

• Integrate functionality from logical_clock_test.cpp and reuse the new fixture there.
• Add one test that causes a write to opLog and check that the result matches the current clusterTime - may be using get lastAppliedOptime() method of replicationCoordinator