Allow database begin_batch functions to fail

[Mic92]

When dealing with local or remote databases than beginning a transaction can fail. Not allowing implementations to return a result and resorting to unwrap() is a non-starter for reliable production code as this will skip Rust's own drop handler in many places and crashes the application.

I have not updated documentation / changelog yet, but would appreciate feedback on the change. I have not run yet any tests.

Description

Notes to the reviewers

Changelog notice

Checklists

All Submissions:

• I've signed all my commits (with ssh)
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[notmandatory]

Concept ACK.

The blockchain sync and database code is completely changing with the bdk_core work that is underway. But until that code is merged into BDK proper I think it makes sense to remove any .unwrap()s we can.

[rajarshimaitra]

Concept + utACK 77123dd

I will go through the code again for a deeper review. And we should probably handle such other unwraps elsewhere too if remains..

[notmandatory]

@Mic92 we had an issue with our CI, please rebase.

[Mic92]

rebased. I made good experience with using bors as a merge bot for things like rebasing and testing against latest HEAD. Github is now adding something similar called merge queue.

[notmandatory]

Thanks for all your work on this but in preparation for our new bdk 1.0.0 work we've gotten to the point where I'd like to hold off on merging any new changes and only merge critical bug fixes to the release/0.27 branch.

The way persistence is done after the bdk_core_staging stuff is merged will be very different so this PR shouldn't apply. But please take a look after #793 is merged, I'd appreciate your review around the new persistence code.

[danielabrozzoni]

Hey, we are in the process of releasing BDK 1.0, which will under the hood work quite differently from the current BDK. For this reason, I'm closing all the PRs that don't really apply anymore. If you think this is a mistake, feel free to rebase on master and re-open!