No global vars

[Ngalstyan4]

• [x] Update usearch API to take a postgres context and pass it to all external retrievers(the context is initialized and passed to usearch iv usearch_init, and it is passed back to postgress in all retriever calls).
• [x] Move extra_dirtied page tracking to per-index-operation context
• [x] Move dirtied header to per-index-operation context and get rid of that last global variable
• [ ] Rethink variables names and, generally, cleanup external_index.c (cleaned up a little, needs more work but let's do that later)

Incorporates the commit from #67 and closes #67

[codecov[bot]]

Codecov Report

Merging #51 (3655eb4) into main (969cc91) will increase coverage by 0.24%. The diff coverage is 89.07%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #51 +/- ## ========================================== + Coverage 82.86% 83.10% +0.24% ========================================== Files 9 11 +2 Lines 887 876 -11 Branches 179 165 -14 ========================================== - Hits 735 728 -7 + Misses 71 67 -4 Partials 81 81 ``` | [Files Changed](https://app.codecov.io/gh/lanterndata/lanterndb/pull/51?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata) | Coverage Δ | | |---|---|---| | [src/hnsw/build.c](https://app.codecov.io/gh/lanterndata/lanterndb/pull/51?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata#diff-c3JjL2huc3cvYnVpbGQuYw==) | `86.73% <75.00%> (-1.15%)` | :arrow_down: | | [src/hnsw/retriever.c](https://app.codecov.io/gh/lanterndata/lanterndb/pull/51?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata#diff-c3JjL2huc3cvcmV0cmlldmVyLmM=) | `83.33% <83.33%> (ø)` | | | [src/hnsw/extra\_dirtied.c](https://app.codecov.io/gh/lanterndata/lanterndb/pull/51?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata#diff-c3JjL2huc3cvZXh0cmFfZGlydGllZC5j) | `86.11% <86.11%> (ø)` | | | [src/hnsw/external\_index.c](https://app.codecov.io/gh/lanterndata/lanterndb/pull/51?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata#diff-c3JjL2huc3cvZXh0ZXJuYWxfaW5kZXguYw==) | `86.22% <89.15%> (+1.19%)` | :arrow_up: | | [src/hnsw.c](https://app.codecov.io/gh/lanterndata/lanterndb/pull/51?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata#diff-c3JjL2huc3cuYw==) | `84.11% <100.00%> (+0.14%)` | :arrow_up: | | [src/hnsw/insert.c](https://app.codecov.io/gh/lanterndata/lanterndb/pull/51?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata#diff-c3JjL2huc3cvaW5zZXJ0LmM=) | `84.41% <100.00%> (-0.40%)` | :arrow_down: | | [src/hnsw/scan.c](https://app.codecov.io/gh/lanterndata/lanterndb/pull/51?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata#diff-c3JjL2huc3cvc2Nhbi5j) | `76.47% <100.00%> (+0.28%)` | :arrow_up: |