SERVER-15062 ReplSetHeartbeatResponse bug fixes from testing
SERVER-15062 ReplSetHeartbeatResponse testing added
SERVER-15145 move pseudo random number generator to ReplicationExecutor from ReplCoordinator
SERVER-14085: Shrink WriteUnitOfWork in DeleteStage::work

This avoids problems for storage engines where committing a transaction
invalidates cursors in that transaction.
Merge remote-tracking branch 'upstream/master'