Add new method to observe ids

[zhirzh]

An observable equivalent to fetchIds that emits ids when records are added or removed. This behaviour can be achieved by chaining observeCount with fetchIds.

class Query {
  ...
  observeIds(): Promise<RecordId[]> {
    return this.observeCount().pipe(switchMap(() => this.fetchIds()))
  }
}

Note that this implementation has followingcaveats:

• requires 2 database queries and data transfers over bridge
• no value is emitted if number of matching records remains unchanged even though matching records changed