This version preserves the order of requests, allows the
client to switch between API versions and introduces a
way to send notifications to the client (see TODO item at:
proc/proc_linux.go:325).
Fixes#523, #571
New API version with better backwards compatibility plus mechanism to
select the API version that a headless instance should use.
Adds service/test/cmd/typecheckrpc.go to type check the RPC interface.