http: release resources when idle

http: release resources when idle

This lets us release old git processes so unlinked packs
(leftover from repacking) can be released.  This may also
be helpful for Xapian as indices get rebuilt for tuning.

For SQLite (msgmap), the there may be no benefit besides
reducing FD pressure.

Followup changes will unify the Inbox and NewsGroup
classes and allow better code-sharing between NNTP and
HTTP classes (as well as the planned POP3 class).