Expose Ingestion API with Inversion of Control

[zaidoon1]

resolve #190

[marvin-j97]

I'm currently massively simplifying the flush logic such that is gets really easy to flush active and sealed memtables in one go by just calling Tree::rotate_memtable followed by Tree::flush while holding a lock.
That also simplifies the flushing logic in fjall.

[marvin-j97]

(Forgot to submit comments)

But as I said, I am currently changing the Flush API which should make the ingestion API much simpler.

(WIP at #194)