fix(database): use switchMap when a list's query changes

[cartant]

Checklist

• Issue number for this PR: Observable-based queries re-emit lists matching previous queries #830
• Docs included?: no
• Test units included?: yes
• e2e tests included?: no
• In a clean directory, npm install, npm run build, and npm test run successfully? yes

Description

This PR adds a failing test and replaces mergeMap with switchMap - which then sees the test pass.

[davideast]

Amazing work again @cartant!