Multi descriptor: Introduce `DatabaseFactory` trait

[evanlinjin]

Description

This is part of #486 to add multi-descriptor wallet support to BDK.

As each DescriptorTracker should have it's own database, and a MultiTracker structure will handle an aggregate of DescriptorTrackers, we need a factory structure that the MultiTracker can use to create Databases for each DescriptorTracker.

Notes to the reviewers

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:

• [x] I've added tests for the new feature
• [x] I've added docs for the new feature
• [x] I've updated CHANGELOG.md