new lookupd server

The lookupd is responsible to maintain the list of resources (jobs) on
different gaffer nodes. It can be used to either find a job on a gaffer node
and launch it or configure it.
`gafferd_cli` -> `cli` & `gaffer_lookupd` -> `lookupd`

esthetic changes. Current wording looked ugly to my eyes.
add lookupd client
useless imports
add a bunch of lookup commands.
add gaffer:lookup command
add find_session method to the registry.

this function allows us to find all jobs registered for a session in the
lookupd server.
add `lookup:session` command