Req Protocol: Free Msg Early if Resends Disabled

[shikokuchuo]

Originally posted in the 'Planning for NNG2.0' Discussion:

Also just to have this on the record. Hopefully can be a 1.x item.

req/rep when NNG_OPT_REQ_RESENDTIME is set to 0 for the req socket, i.e. no resends, free the msg set on the context i.e. req0_ctx->req_msg early when the send is complete without waiting for the reply.

Originally posted by @shikokuchuo in #1781 (comment)

This is for use in a typical RPC context where I do not use NNG-level retries, so it serves no purpose to hang onto the sent message. This helps to minimize total memory usage when many async requests are underway.

I have a solution which I will propose in a PR.