This PR adds support for the lock service API:
https://github.com/etcd-io/etcd/blob/master/Documentation/dev-guide/api_concurrency_reference_v3.md#service-lock-etcdserverapiv3lockv3lockpbv3lockproto
https://github.com/etcd-io/etcd/blob/master/etcdserver/api/v3lock/v3lockpb/v3lock.proto

Two methods are added to the client via the LockAPI class:

• lock(self, name, lease=0)

  Lock acquires a distributed shared lock on a given named lock.
  On success, it will return a unique key that exists so long as
  the lock is held by the caller. This key can be used in
  conjunction with transactions to safely ensure updates to etcd
  only occur while holding lock ownership. The lock is held until
  Unlock is called on the key or the lease associate with the
  owner expires.
  

• unlock(self, key):

  Unlock takes a key returned by Lock and releases the hold on
  lock. The next Lock caller waiting for the lock will then be
  woken up and given ownership of the lock.