keepalive: use collections.defaultdict for host map
Cleaning up the code as part of debugging Python 3 issues.
Differential Revision: https://phab.mercurial-scm.org/D5718
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state