Simplify pythia.Conn implementation

This also fixes zombie goroutines on connection close.
backend/pool: Proper shutdown

On shutdown, the Pool now aborts all remaining jobs.

Note that we rely on the fact that we can just send messages to a closed
connection, and that this will not block (it will generate an error
however). This is probably not entirely race-free however.
devtools: Setup script for Go development
backend: Add Queue component