Replace use of Gee.TreeSet in Util.Cache.Lru

We're getting crashes (#398, #609) in Gee.TreeSet for the LRU cache when
removing values to update the last used time for an existing cache.
Since Gee isn't currently being activaly maintained, replace use of
that class with something else.
4 jobs for mjog/lru-crash-3-34 in 29 minutes (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #503961
ubuntu

00:13:22

 
  Test
passed #503962
flatpak

00:14:48

 
  Deploy
passed #503963
review

00:00:49

failed #503964
allowed to fail manual
stop_review

 
Name Stage Failure
failed
stop_review Deploy There has been a missing dependency failure
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