*: Update references to code moved to the api/ dir.

Follow up to file-moves done in the previous commit.

The commit contains purely mechanical consequences of execution (apart
of scripts/genproto.sh):

  % find ./ -name '*.go'  | xargs sed --follow-symlinks -i 's|v3/etcdserver/api/v3rpc/rpctypes|v3/api/v3rpc/rpctypes|g'
  % find ./ -name '*.go'  | xargs sed --follow-symlinks -i 's|v3/version|v3/api/version|g'
  % find ./ -name '*.go'  | xargs sed --follow-symlinks -i 's|v3/mvcc/mvccpb|v3/api/mvccpb|g'
  % find ./ -name '*.go'  | xargs sed --follow-symlinks -i 's|v3/etcdserver/etcdserverpb|v3/api/etcdserverpb|g'
  % find ./ -name '*.go'  | xargs sed --follow-symlinks -i 's|v3/etcdserver/api/membership/membershippb|v3/api/membershippb|g'
  % find ./ -name '*.go'  | xargs sed --follow-symlinks -i 's|v3/auth/authpb|v3/api/authpb|g'

  % find ./ -name '*.proto' -o -name '*.md'  | xargs -L 1 sed --follow-symlinks -i 's|/mvcc/mvccpb/kv.proto|/api/mvccpb/kv.proto|g'
  % find ./ -name '*.proto' -o -name '*.md'  | xargs -L 1 sed --follow-symlinks -i 's|/auth/authpb/auth.proto|/api/authpb/auth.proto|g'
  % find ./ -name '*.proto' -o -name '*.md'  | xargs -L 1 sed --follow-symlinks -i 's|/etcdserver/api/membership/membershippb/membership.proto|/api/membershippb/membership.proto|g'

  I also modified manually paths in scripts/genproto.sh.

  % go fmt ./...