ftp: Refactor command queue into sequential executor

The FTP door uses a custom CommandQueue to provide the
ability to sequentially process commands using a shared
thread pool. This patch refactors this code into a more
generic SequntialExecutor class. It implements the
ExecutorService interface and is semantically similar to
a single thread executor. However the thread used for
processing tasks is allocated on demands from a shared
Executor.

The selective enabling of interrupts has been removed
from the FTP door. This was a workaround for bad exception
handling in old versions of the door. The workaround is
not easily ported to SequentialExecutor and is therefore
removed.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6325/