Handle async exceptions correctly in bracket and finally.

[dterei]

When I used the latest simple with povo ran into an issue where withConnection wasn't correctly running it's finally / bracket clause to return the connection back to the MVar. Investigating showed I needed to mask async exceptions for finally and bracket to behave as expected.

[alevy]

And mask needs to wrap the entire operation (including aquire etc), which is why we can't just do it in onException?

[dterei]

Yeah, you essentially want the operations to be atomic, so treat mast like taking a lock over the whole thing but we can use the restore callback to re-enable exceptions in just a sub-computation.