Future not cancelled if interrupted in MemcachedCache

See [checklist](https://medium.com/@leventov/code-review-checklist-java-concurrency-49398c326154), item 11.3. Similar problems could exist in other places in the codebase.

Start by locating MemcachedCache and reading checklist item 11.3 from the linked concurrency review. Trace how interruption is handled when the cache operation returns a Future, then inspect similar code paths as suggested by the issue. Done means interrupted work is cancelled consistently and the relevant behavior is covered by existing or added tests.

Written by the indexing model from the issue text.