PIP-1319: Update to the latest etcd client

[horkhe]

Since the etcd.Client package has changed and the client reference is exposed to the world this is an API breaking change, hence version 2 of this module was introduced.

Also:

• Stubs were re-genrated with the latest protobuf and grpc versions;
• Deprecated ConsistentHash was removed;
• HashBytes64 is replaced with HashString64 to avoid unsafe conversions that is reported by go vet since v1.16 cmd/vet: warn about variables/values of type reflect.{Slice,String}Header golang/go#40701 (comment).

[thrawn01]

Since we are currently on pre-release status Release v1.0.0-rc.9 (Release Candidate 9). I wonder if we should just update the import paths to /v1 instead of jumping to /v2.

[horkhe]

I do not think we can get around with adding v1 to the version path, current top tags are already v1.x.x.