rfc3: add 'matchtag' field to PROTO

This 8-bit field is used to correlate request - response message
pairs, allowing up to 255 requests to be outstanding in one handle
at any given time.

Matching requests and responses based on topic strings (as in our
prototype code) artificially limits the number of outstanding requests
per handle to one per unique topic string.
Merge pull request #14 from garlick/cmbd-matchtag

rfc3: add 'matchtag' field to PROTO