fix: lock lru-cache to v6.0.0

[wschurman]

Why

lru-cache v7.x is optimized for non-TTL caching: isaacs/node-lru-cache#208. Instead, it's optimized for GC minimization and does this by pre-allocating the entire data structures at time of construction: https://github.com/isaacs/node-lru-cache/blob/main/index.js#L119

For our use case, we know we'll be using a short TTL so we're not too worried about memory or GC being an issue. In fact, pre-allocating all the data structures ahead of time drastically slows down our server (especially our test suite).

The old implementation is more aligned with our use case since it doesn't do any allocation ahead of time: v6.0.0 implementation: https://github.com/isaacs/node-lru-cache/blame/dcd2384fd3321d86dc6ebb3cee3e9601727d1555/index.js

Other (current) libraries were investigated as alternatives but seemed a bit heavy:

• The fixed-size allocation in v7 restricts many previously possible use cases isaacs/node-lru-cache#208 (comment)
• https://github.com/node-cache/node-cache
  What we really need is a TTL LRU cache, and v6.0.0 of lru-cache provides that adequately.

How

Reverse course and lock at 6.0.0, install correct typedefs.

Test Plan

Wait for CI.

[codecov]

Codecov Report

Merging #170 (e15752d) into master (02d2f24) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #170      +/-   ##
==========================================
- Coverage   96.15%   96.14%   -0.01%     
==========================================
  Files          80       80              
  Lines        1975     1972       -3     
  Branches      211      209       -2     
==========================================
- Hits         1899     1896       -3     
  Misses         75       75              
  Partials        1        1              

Flag	Coverage Δ
integration	96.14% <100.00%> (-0.01%)	⬇️
unittest	96.14% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...apter-local-memory/src/GenericLocalMemoryCacher.ts	100.00% <100.00%> (ø)

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