fix(db): inherit generics in valueChanges interface

[Maistho]

Checklist

• Issue number for this PR: valueChanges does not inherit type of list/object #1214 (required)
• Docs included?: not required
• Test units included?: no
• In a clean directory, npm install, npm run build, and npm test run successfully?: yes

[markgoho]

Looks like a simple fix for redundant typing

[davideast]

Thanks @Maistho!