Thread through the operation wall-clock time for oplog entries

All oplog entries contain a wall value, which specifies the wall-clock time when the operation was logged. Currently this time is assigned at the time of generating the oplog entry. However, there is a use-case, which requires that value to be synchronized between the config.transactions collection and the oplog. Because of this, we should make it an argument to logOp and have the caller generate it.