ctl: change has_switch_id to has_client_id

Better naming.
ctl: Add switch_id_of_client to OpenFlow0x01.Controller

There has to be a way to retrieve the switch_id associated with a
connection. The client of the OpenFlow0x01.Controller module could
maintain this mapping, but having the module provide it is more
convenient, especially since it's already maintaining the mapping.
ctl: add client_id_of_switch to OpenFlow0x01.Controller

Complete the invertible map between client_id and switch_id. There
should really only be one. But that's for another pull request.
nib: use ocaml-toplogy instead of custom State module
nib: expose nib to users of controller
Merge pull request #99 from frenetic-lang/nib

Use ocaml-topology as the basis for a NIB