etcdserver: use hex for cluster and machine id

Continue using hex everywhere. Including here.

TODO: cleanup the printing of the structs which currently have decimal
to/from:

`{Type:MsgAppResp To:9973738105406047488 From:17050684879817348455 T...`
Merge pull request #1362 from philips/consistent-hex-use

etcdserver: use hex for cluster and machine id