checked exceptions in stores

[marcinczenko]

While working on BitTorrent stuff and trying to keep exceptions checked, I continuously hit the wall of set of "stores" which mostly do not use anything but .async..

This PR adds checked exceptions to all stores and dependent abstractions.

To keep the PR still maintainable it focuses on the "stores", thus it has pretty much a minimal set of changes to get them compile.

Most of the changes are easy and syntactical, but there were a few exceptions where it was a big pain to move forward.
It all boils down to AsynIter, most probably created before async: (raises: [CancelledError]) era, that makes checking exceptions much harder where it is used.

The only way to get it clean was for me to create a version of AsyncIter (called SafeAsyncIter) that uses Results and only ever allows for async: (raises: [CancelledError]). I applied it to listBlocks and getBlockExpirations and doing this I could remove problematic situations (especially in block maintenance). SafeAsynIter has the same API as AsynIter and requires very little work (pretty much syntactical) to use it in place of AsyncIter, which is still used in some other places, which can be dealt with in a separate PR to keep this one small.

To get some insights about challenges that you may encounter when working with checked (raising) exception, you may like to check the following note: https://publish.obsidian.md/bittorrent/10+Notes/Async+checked+(raising)+exceptions.