isMaster lastWrite field

When run on a replicaset, the isMaster command now returns a new top level field called "lastWrite". lastWrite contains the "opTime" and "lastWriteDate" which is the optime and date of the last applied write on that server. If --enableMajorityReadConcern is specified, "majorityOpTime" and "majorityWriteDate" will also appear. They return the optime and date of the last committed snapshot, which is the latest optime and date of the writes that would be returned if a readConcern majority read were issued against that server.

"lastWrite" : {

		"opTime" : {

			"ts" : Timestamp(1462197355, 1),

			"t" : NumberLong(1)

		},

		"lastWriteDate" : ISODate("2016-05-02T13:55:55Z"),

		"majorityOpTime" : {

			"ts" : Timestamp(1462197355, 1),

			"t" : NumberLong(1)

		},

		"majorityWriteDate" : ISODate("2016-05-02T13:55:55Z")

	},