We are going to want rpc request to be immutable so that
we can take a copy and put it aside to be processed later.
Currently the tcp code writes the response into the same
buffer as the request, thus corrupting the request.
With this patch, the response goes after the request.  There should
always be enough room as large reqeusts (Write) has small
responses, and large responses (read) are for small requests.

buflen is changed for requests to record the length of the
request.  It already gets reset for each new request.