Add query comment to oplog tailer for troubleshooting/profiling/QAN

[timvaillancourt]

Add query comment to oplog tailer for troubleshooting:

rs:PRIMARY> db.system.profile.find({}, {command:1}).limit(1).pretty()
{
	"command" : {
		"find" : "oplog.rs",
		"filter" : {
			"op" : {
				"$ne" : "n"
			},
			"ts" : {
				"$gte" : Timestamp(1531909597, 0)
			}
		},
		"skip" : 0,
		"batchSize" : 100,
		"comment" : "github.com/percona/mongodb-backup/internal/oplog.(*OplogTail).tail()",
		"oplogReplay" : true,
		"$db" : "local"
	}
}