wireproto: define and implement responses in framing protocol
Previously, we only had client-side frame types defined. This commit
defines and implements basic support for server-side frame types.
We introduce two frame types - one for representing the raw bytes
result of a command and another for representing error results.
The types are quite primitive and behavior will expand over time.
But you have to start somewhere.
Our server reactor gains methods to react to an intent to send a
response. Again, following the "sans I/O" pattern, the reactor
doesn't actually send the data. Instead, it gives the caller a
generator to frames that it can send out over the wire.
Differential Revision: https://phab.mercurial-scm.org/D2858
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