two %d really needs %zu

a missing ui_schedule_redraw on unkill_tab means that the tabbar is not rendered when calling tab-undo-close when there is only one tab.

delete pages that are older than one hour, and scan the cache only once every five minutes. The previous defaults were a bit too much conservative.

instead of keeping the struct mcache that's private to mcache.c and used only once, just define its fields as (static) variables. While here, mark as static also gemtext_prefixes.