cache: fix race when clearning a cache

[vmg]

Hiii!! Thanks for Ristretto, it's very nice! I've found an actual bug when wiring it up as a cache backend for Vitess; it can cause goroutines to block.

Fix and reproduction recipe attached!

When clearing a cache while other goroutines are actively `Wait`ing for
it, the waiting goroutines can become blocked indefinitely if one of the
cache items is flushed by the `Clear` code instead of by the background
worker, because the `Clear` code is not marking waitgroups as done.

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[karlmcguire]

Great catch!

Reviewed 1 of 2 files at r1, 1 of 1 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jarifibrahim, @manishrjain, and @martinmr)