implement deletion vector handling in index scan

[chebbyChefNEQ]

closes #916

Implement deletion vector handling in PQIndex, where all vector index are stored.

TODO:

• [ ] add tests
• [ ] add TODO comments in places we could have potential performance gains

[chebbyChefNEQ]

TL;DR:

• Seems to have very little latency impact
• We should also test query throughput than just latency.

after picking in #960 the difference of index scan performance pre/post this PR is negligible. Any perf impact introduced is smaller in magnitude than noise.

The impact maybe become more pronunciated in a environment where IO latency is high, say, EC2 instance with EBS, which is a remote disk.

Since this change introduces more IO, I'd hypothesis that there could be a hit to the query throughput

[eddyxu]

Seems to have very little latency impact

is this for scanning or the ANN search?

[chebbyChefNEQ]

is this for scanning or the ANN search? ann search. The bench mark uses nearest.