pool: Rename and parameterize execution services

The execution services are inconsistenly named. Some are called
ExecutorService, some ExecutionService and the NFS one is misspelled
as ExcecutionService.

The patch renames these as variations of TransferService, which IMO
better capture the purpose of the interfaces.

I parameterized the interfaces to avoid coupling the interface
to any particular mover implementation, yet allow implementations to
access mover specific properties.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5437/