Clean up fds of segments we delete (during compaction)
Clean up fds of segments we delete (during compaction)

When we delete a segment, let's close its fd as well.
Note as well wasting the fd, this was forcing the
filesystem to preserve the deleted file until we exited.

I noticed roughly 20 open fds of deleted files when
attic saved 10G of data.

RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,edgimar/borg,RonnyPfannschmidt/borg,edgewood/borg,level323/borg,edgimar/borg,edgewood/borg,level323/borg,edgimar/borg,mhubig/borg,raxenak/borg,edgimar/borg,raxenak/borg,mhubig/borg,level323/borg,mhubig/borg,edgewood/borg,raxenak/borg,raxenak/borg,edgewood/borg