client: Move client specific code (protos, version) to the api/
directory. Thanks to this change /client directory will not need to depend on
the server code. In next commits we make "/api" a module on its own.
Mechanical consequences of execution:
% git mv version/version.go api/version
% git mv etcdserver/api/v3rpc/rpctypes api/v3rpc
% git mv mvcc/mvccpb api/
% git mv etcdserver/etcdserverpb api/
% git mv auth/authpb api/
% git mv etcdserver/api/membership/membershippb api/
