low-impact - lrucache code appears to leak fds without closing them

I haven't verified this by testing, but the lrucache code looks a bit broken.

Possible fix is in my lrucache branch - see e3f671c.  I based it on my PR #339 / commit 57845c0 "Clean up fds of segments we delete (during compaction)", so I guess it's blocked on that PR.  It'll teach me to keep better PR discipline :).

https://github.com/sourcejedi/attic/tree/lrucache
