'connection_state Implementation.t is something that knows how to respond to one
query, given a
'connection_state. That is, you can create a
Implementation.t by providing a function which takes a query *and* a
'connection_state and provides a response.
The reason for this is that RPCs often do something like look something up in a master
structure. This way,
Implementation.ts can be created without having the master
structure in your hands.