Otherwise code in stream_session.cc
_transfers.emplace(cf_id, stream_transfer_task(shared_from_this(), cf_id)).first;
will copy the stream_transfer_task and in turn copy the
outgoing_file_message which is not copyable due to the semaphore member.
Thanks avi for figuring this out.
I tried our lw_shared_ptr, the compiler complained endless usage of
incomplete type stream_session. I can not include stream_session.hh
everywhere due to circular dependency.
For now, I'm using std::shared_ptr which works fine.
Each outgoing_file_message might contain multiple mutations. Send them
one mutation per RPC call (using frozen_mutation), instead of one big
outgoing_file_message per one RPC call.
