Remove generic `D: Database` from `CoinSelectionAlgorithm`

[evanlinjin]

Fixes #281

Description

Since CoinSelectionAlgorithm is a trait, if the database is really needed, one can create a struct that contains a database field which implements the CoinSelectionAlgorithm trait.

Most CoinSelectionAlgorithm implementations do not require a database.

Changelog notice

The CoinSelectionAlgorithm trait no longer depends on a database.

Checklists

All Submissions:

• [x] I've signed all my commits
• [x] I followed the contribution guidelines
• [x] I ran cargo fmt and cargo clippy before committing

New Features:

~* [ ] I've added tests for the new feature~

• [x] I've added docs for the new feature
• [x] This pull request breaks the existing API (it removes a single input from a trait's method)

[evanlinjin]

To do

• [ ] LargestFirstCoinSelection should use Ref<D> as a field (otherwise it will be inconvenient/confusing to use). I've converted this to draft again.
• [ ] Write an example of using LargestFirstCoinSelection with Wallet.