Fixing null set with RTDB and more

[jamesdaniels]

Checklist

• Issue number for this PR: AngularFireDatabase doesn't execute functions when an empty list should be returned #1220, snapshotChanges with orderByChild does not return data in correct order #1246, etc.
• Docs included?: yes
• Test units included?: yes
• In a clean directory, npm install, npm run build, and npm test run successfully? yes

Description

Working on closing out some of the issues and cleaning up the code with the 5.0 of the RTDB Service.

• Once now closes out the subscription
• Handle multiple subscriptions to the AngularFireDatabase
• Empty set should return null
• Better batching of first load, using once (needed step to support Universal)
• Handle ordered child_added events
• Don't error out on unsubscribe of a once
• Clean up the tests to use take(...) and add(...)
• Cleaning up the types

TODOS:

• Clean up the code some more
• Figure out why I needed to go online for some of the tests, reduce that as much as possible
• Test sorting some more

Code sample

let items = [...];
let count = 0;
db.list('items').snapshotChanges().subscribe(snapshots => console.log(count += 1, snapshots));
await ref.set(items);
await ref.push({ name: '...' });
await ref.child(items[0].key).update({ name: '...'});
await ref.child(items[1].key).remove()
await ref.child(items[0].key).setPriority('...');

LOG: 1, [Object{payload: ..., type: 'value', prevKey: undefined, key: '0'}, Object{payload: ..., type: 'value', prevKey: undefined, key: '1'}, Object{payload: ..., type: 'value', prevKey: undefined, key: '2'}]
LOG: 2, [Object{payload: ..., type: 'value', prevKey: undefined, key: '0'}, Object{payload: ..., type: 'value', prevKey: undefined, key: '1'}, Object{payload: ..., type: 'value', prevKey: undefined, key: '2'}, Object{type: 'child_added', payload: ..., prevKey: '2', key: '-KwM4ThL2evnTjT-KHhV'}]
LOG: 3, [Object{type: 'child_changed', payload: ..., prevKey: null, key: '0}, Object{payload: ..., type: 'value', prevKey: undefined, key: '1'}, Object{payload: ..., type: 'value', prevKey: undefined, key: '2'}, Object{type: 'child_added', payload: ..., prevKey: '2', key: '-KwM4ThL2evnTjT-KHhV}]
LOG: 4, [Object{type: 'child_changed', payload: ..., prevKey: null, key: '0'}, Object{payload: ..., type: 'value', prevKey: undefined, key: '2'}, Object{type: 'child_added', payload: ..., prevKey: '2', key: '-KwM4ThL2evnTjT-KHhV'}]
LOG: 5, [Object{payload: ..., type: 'value', prevKey: undefined, key: '2'}, Object{type: 'child_added', payload: ..., prevKey: '2', key: '-KwM4ThL2evnTjT-KHhV'}, Object{type: 'child_changed', payload: ..., prevKey: null, key: '0'}]
LOG: 6, [Object{payload: ..., type: 'value', prevKey: undefined, key: '2'}, Object{type: 'child_added', payload: ..., prevKey: '2', key: '-KwM4ThL2evnTjT-KHhV'}, Object{type: 'child_changed', payload: ..., prevKey: '-KwM4ThL2evnTjT-KHhV', key: '0'}]