return revision when put_if_not_exists() succeed

[PingHao]

Current implement of put_if_not_exists() only return bool indicate succeed or not, I think it's make more useful to return revision of key, for example, after put_if_not_exits(KEY), user add watch of KEY to monitor if it's been delete or not. without revision, it always has the chance that the next watch call didn't catch all event since PUT, unless use revision as parameter.

with revision, a better user code can be write as

status,revision = put_if_not_exists(client,KEY,VALUE)
# delay can happen between two calls
client.add_watch_callback(TEST_KEY,watch_cb,start_revision=revision+1)

I tried to write my own impl, but it end with unwrap RPC message layers, (resp.response_put.header.revision) not quite good.

def put_if_not_exists(client, key, value, lease=None):
    status, responses = client.transaction(
        compare=[
            client.transactions.version(key) == 0
        ],
        success=[
            client.transactions.put(key, value, lease)
        ],
        failure=[],
    )
    revision = 0
    if status:
        for resp in responses:
            revision = resp.response_put.header.revision
    return status, revision

[PingHao]

FYI, this is a gist I wrote about leader election recipe compatible with "etcdctl elect" command, in there it use put_not_exist() return revision.
https://gist.github.com/PingHao/c9ab07b6e82c0887cce970b10466e307