Avoid using variable_set() for storing cache flush times.

fwiw I went a different way in memcache, adding memcache-specific versions of variable set so that it's write through.

There were two main reasons for this:

- storing it in memcache risks losing it if there's an eviction or partial restart of a memcache cluster. That's not an issue for mongo though.
- having an extra request to memcache to check expiry had a noticeable performance impact on raw page caching performance. This makes sense since you're talking about only a handful (or in D7 potentially 1) requests - so adding one is not small in that context.

Removed separate issue info

This was a good idea, but the D7 version of the module won't evolve now that Drupal core is already at 8.5.

Might make sense after rethinking what it could mean on D8.