Implement lock method and other methods from etcdctl

[chingologram]

I think that, as a general guideline, and for consistency/transparency, the client should aim to provide the same functionality as etcdctl.

Here the list of methods that are not currently implemented.

• alarm disarm
• alarm list
• check datascale
• check perf
• compaction
• defrag
• elect
• endpoint hashkv
• endpoint health
• endpoint status
• lease grant
• lease keep-alive
• lease list
• lease revoke
• lease timetolive
• lock
• make-mirror
• member add
• member list
• member promote
• member remove
• member update
• migrate
• move-leader
• snapshot restore
• snapshot save
• snapshot status
• txn
• version
• watch