Improve PQ DX: Remove multi-step process (import, pause, fit, continue importing)

[etiennedi]

Currently to import with PQ multiple steps are required:

1. Import a specific fraction of the dataset
2. Pause import to enable PQ
3. Poll API to wait for fitting to complete
4. Continue import

Instead (by making use of async importing) the user should be able to turn on PQ before they even start importing. Then they just import and Weaviate will figure out the rest.

[trengrj]

Completed in #3771