msg/async/rdma: Extract sockets stuff from RDMAStack.h

msg/async/rdma: Extract sockets stuff from RDMAStack.h

This is a preparation commit, in order to make review easier. In this
commit I move code from RDMAStack.h into the new file
RDMAConnectedSocketImpl.h - without changing the code.

In the next commit, the actual logic changes will be done and socket
classes will be split into a base RDMAConnected classes and child
classes with TCP connection establishment specific code.

Issue: 995322
Change-Id: I639fda490a6fbd02addb95d3158c5ac1e7390ef0
Signed-off-by: Amir Vadai <amir@vadai.me>