Ensure crossbeam-epoch to run GC when dropping a cache

[tatsuya6502]

Fixes #383.

This PR ensures crossbeam-epoch to run GC (run the deferred_fns in the global bag) when dropping a moka cache. This should help the entries, who are currently or previously in the cache, to be dropped.

We have been struggling this crossbeam-epoch spec for years. I added a comment to our notes about this issue.

[tatsuya6502]

As of the latest commit 6b79f58, some of the CI jobs are failing.

It seems the following configurations causes one of the new tests sync::cache::tests::ensure_gc_runs_when_dropping_cache sometimes to fail.

$ cargo test --release --no-default-features --features 'sync, atomic64'
$ cargo test --release --no-default-features --features sync

And the following configurations seem never fail:

$ cargo test --release --features sync

$ cargo test --release --no-default-features --features 'future, atomic64'
$ cargo test --release --no-default-features --features future
$ cargo test --release --features future

[tatsuya6502]

Hmm. Additional commits may have stabilized the flaky test. I am going to merge this PR and continue watch these jobs.

[tatsuya6502]

Merging.