Roughly script up a localized postgres
Support several new message types, and give in and use threads instead of coroutines so that I can handle network and REPL traffic simultaneously.

This commit also enhances the struct wrappers to support C strings,
 because several of the medium sized and larger postgres messages have subfields
 with unlabelled widths--you just have to scan for \0 which took a long time to sort out.
Next: rewriting the scratches in all the parse() lines to use the enhanced support for C-strings
Handle sockets falling over
Finish writing the subtypes of CommandComplete
Switch over to using pgpack and pgunpack everywhere.
Write a ReplicationClient, which is like a PgClient (and uses it) except it dumps to replication mode from the start (as the protocol insists you do) and then speaks its own language embedded in PgClient CopyData messages

I think this is the cleanest way right now to disentangle the several layers of protocols that is the postgres protocol.