Revert to fa_cache for node retriever

[codecov[bot]]

Codecov Report

Merging #203 (0c113c1) into main (0c0aceb) will increase coverage by 0.08%. The diff coverage is 94.11%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #203 +/- ## ========================================== + Coverage 82.97% 83.05% +0.08% ========================================== Files 17 18 +1 Lines 1204 1216 +12 Branches 258 261 +3 ========================================== + Hits 999 1010 +11 Misses 81 81 - Partials 124 125 +1 ``` | [Files](https://app.codecov.io/gh/lanterndata/lantern/pull/203?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata) | Coverage Δ | | |---|---|---| | [src/hnsw/external\_index.c](https://app.codecov.io/gh/lanterndata/lantern/pull/203?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata#diff-c3JjL2huc3cvZXh0ZXJuYWxfaW5kZXguYw==) | `91.09% <100.00%> (ø)` | | | [src/hnsw/retriever.c](https://app.codecov.io/gh/lanterndata/lantern/pull/203?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata#diff-c3JjL2huc3cvcmV0cmlldmVyLmM=) | `73.33% <100.00%> (-0.87%)` | :arrow_down: | | [src/hnsw/fa\_cache.h](https://app.codecov.io/gh/lanterndata/lantern/pull/203?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata#diff-c3JjL2huc3cvZmFfY2FjaGUuaA==) | `92.30% <92.30%> (ø)` | |