Remove CmdID from InternalRaftCommand and pass it to raft separately.

Multiraft needs a command identifier so it can track pending commands
and resubmit them when an election has caused them to be dropped.

@spencerkimball @cockroachdb/developers 
RandStrip actually leaves a lot of bits on the floor and is extraordinarily inefficient if all we're trying to do is generate 16 bytes. Let's just get two random int64s and append them using:

    return encoding.EncodeUint64(encoding.EncodeUint64(uint64(rand.Int63)), uint64(rand.Int63))