Add Etcd v3 protocol support via api gRPC-gateway (#1162)

The only python-etcd3 client working directly via gRPC still supports only a single endpoint, which is not very nice for high-availability.

Since Patroni is already using a heavily hacked version of python-etcd with smart retries and auto-discovery out-of-the-box, I decided to enhance the existing code with limited support of v3 protocol via gRPC-gateway.

Unfortunately, watches via gRPC-gateway requires us to open and keep the second connection to the etcd.

Known limitations:
* The very minimal supported version is 3.0.4. On earlier versions transactions don't work due to bugs in grpc-gateway. Without transactions we can't do atomic operations, i.e. leader locks.
* Watches work only starting from 3.1.0
* Authentication works only starting from 3.3.0
* gRPC-gateway does not support authentication using TLS Common Name. This is because gRPC-proxy terminates TLS from its client so all the clients share a cert of the proxy: https://github.com/etcd-io/etcd/blob/master/Documentation/op-guide/authentication.md#using-tls-common-name

setup.py

@@ -22,7 +22,7 @@ AUTHOR_EMAIL = '[email protected], [email protected], alexk
 KEYWORDS = 'etcd governor patroni postgresql postgres ha haproxy confd' +\
     ' zookeeper exhibitor consul streaming replication kubernetes k8s'
 
-EXTRAS_REQUIRE = {'aws': ['boto'], 'etcd': ['python-etcd'], 'consul': ['python-consul'],
+EXTRAS_REQUIRE = {'aws': ['boto'], 'etcd': ['python-etcd'], 'etcd3': ['python-etcd'], 'consul': ['python-consul'],
                   'exhibitor': ['kazoo'], 'zookeeper': ['kazoo'], 'kubernetes': [], 'raft': ['pysyncobj']}
 COVERAGE_XML = True
 COVERAGE_HTML = False