Clean up rpc.Conn.receive

[lthibault]

This is a refactor of rpc.Conn for readability. There are no functional changes.

Clean-up is concentrated in two places:

1. The switch statement in Conn.receive is tidied up by having each case call out to a dedicated "handler" method.
2. The reader thread is moved into its own task, allowing it to trigger task shutdown on its own. This in turn lets us implement receiver as a simple range loop over a channel.

A follow-up effort will focus on abstracting tasks & connection lifecycle.

[zenhack]

This broadly looks good to me, modulo the one comment about reader().