server: irminsule interface: replace the pair of mvars with threads

Previously we decoupled the main server from the backend by using
RPC over a pair of mvars. It's simpler just to parse the cmdline
arguments and create the appropriate backend, and then Lwt.wakeup
a pair of threads everyone else just waits for.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>