Implement ClearAsync in data sources

[manandre]

Fixes #4499

[manandre]

@roji and @vonzshik, your comments are welcome.

[roji]

@manandre just to confirm, you're waiting on our review for this right?

If not, can you please rebase and we'll do our best to review and merge ASAP? Not sure this will happen for 8.0 at this point, but we can always try.

[manandre]

@roji I have just rebased. However, I am still not fully confident around the lock replacement in the Close method of the NpgsqlConnector class... IMHO No hurry is required here, it can wait for next version if safer.

[NinoFloris]

Same here, let's aim to get this into a patch for 8.0. Thanks @manandre