ftp: Fix race that lead to erroneous error message in log file

Removes the following error message:

05 dec 2013 14:35:34 (httpd) [door:httpd@dCacheDomain:1386250529077 door:httpd@dCacheDomain:1386250530581 door:httpd@dCacheDomain:1386250530980 door:httpd@dCacheDomain:13
86250531049 door:httpd@dCacheDomain:1386250531677 door:httpd@dCacheDomain:1386250531744 door:httpd@dCacheDomain:1386250533468 door:httpd@dCacheDomain:1386250533660 door:h
ttpd@dCacheDomain:1386250534760] Cannot abort transfer that already completed: 451 Aborting transfer due to session termination

This error is a result of a race in which door shutdown and
the transfer termination sequence conincide. The patch
rearranges the termination sequence to mark the transfer
as completed before providing the final reply to the client.
The patch also removes the error logging.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6320/