Accumulated enhancement to etcd-cpp-apiv3.

[sighingnow]

hi @CsatariGergely ,

I'm so happy to see this repository get updated by the author team! And so many developers that invokes in etcd API benefit from this project.

However this package hasn't been updated since 4 years ago, and we do do a better job to make this project more helpful for C++ developers.

This PR includes our efforts to make the library work for recently version of etcd, c++ compilers, cmake, and provides better experiments for installment and deployment.

Detailedly, this pull request includes:

• update protobuf definition to latest version in etcd repo
• better cmake integration, includes:
  • generate protobuf files on-modify to accelebrate building process
  • better facilities to install etcd-cpp-apiv3 as a "package" that could be found by on-top programs.
• implements the etcd "lock" api
• fixes bugs in the "WatchAction"
• load balance (in gRPC) over multiple etcd endpoints.

Since this project is the recommanded way to connect to etcd in C++ on the etcd website, I'm looking forward to getting this PR merged and help more people. If you have any question about getting this one merge, feel to let me know.

Best regards,
Tao

[sighingnow]

Besides, if it is possbile I'm very interested in co-maintainning this project :)