Keep only one global RPC runner in Ganeti context

Instead of having one RPC runner per mcpu processor this will keep only
one instance as part of the masterd-wide Ganeti context. Upcoming
patches will change the RPC runner to report pending requests to the
lock manager.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>