webdav: Kill mover in case transfer times out

Rather than leaving movers to timeout (and filling a transfer slot
in the meantime), this patch sends a notification to pools to kill
the mover in case the transfer times out in the door.

This approach is already used by other doors. I submit this for
inclusion in 2.10 as more sites will begin to see issues with the
webdav door as the use of HTTP increases.

Target: trunk
Request: 2.10
Require-notes: yes
Require-book: no