CollectionFetchJob: ensure result() is the last signal emitted.

If we simply use delayedEmitResult() instead of emitResult() it is already
ensured that all collections are emitted before result, the whole flushing
not necessary. This also fixes the recursive fetch, where the collections
were not emitted before result, which is IMO a bug.
I added the required tests to ensure the job behaves like it should.

REVIEW: 118329