[rpc] nano-optimizations on RPC feature flags

Avoid memory re-allocations and allow for optimizations (e.g., memcpy())
when copying elements between std::vector and protobuf's RepeatedField.

This changelist doesn't contain any functional modifications.

Change-Id: I77ec723ad8aed3dcc18e3a653740ddf1da95fb2c
Reviewed-on: http://gerrit.cloudera.org:8080/24221
Reviewed-by: Abhishek Chennaka <achennaka@cloudera.com>
Tested-by: Alexey Serbin <alexey@apache.org>