SERVER-15195 Implement getHostsWrittenTo in ReplicationCoordinatorImpl
SERVER-14567 Implement checkIfWriteConcernCanBeSatisfied in ReplicationCoordinatorImpl
SERVER-14445 Remove waitUpToOneSecondForOptimeChange from ReplicationCoordinator
SERVER-15248 Introduce ScatterGatherAlgorithm interface and ScatterGatherRunner.

The ScatterGatherAlgorithm interface describes operations that a scatter-gather
process must perform.  ScatterGatherRunner uses a ReplicationExecutor to
execute a ScatterGatherAlgorithm.
Merge remote-tracking branch 'upstream/master'