Merge remote-tracking branch 'kali/feature/in-memory-store' into develop
minimal regression tests
fix expunge deferreds so they wait
fix memoized call returning always None
Merge remote-tracking branch 'refs/remotes/kali/feature/regression-tests' into develop
Fix the fallback for the memoized call for bodies/content.

Changed to "empty" to consider empty strings too.
Merge remote-tracking branch 'refs/remotes/kali/bug/fix-fetch-bodies' into develop
Flush IMAP data to disk when stopping. Closes #5095.