feat: BREAKING no more leveldown

[sharafian]

The leveldown module packaged with the connector is unused in almost all cases and causes a plethora of build errors and other issues when installing the connector. This PR removes the leveldown store and the memdown store and replaces them with a memory store that is functionally the same as the memdown one but doesn't require a dependency.

This is a breaking change. If there's interest in keeping the leveldown store around it could be moved into a standalone module but AFAIK nobody is still using it. Even the balance middleware is typically not used in production so on most deployments no store is being used.

[codecov-io]

Codecov Report

Merging #495 into master will increase coverage by 0.11%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #495      +/-   ##
==========================================
+ Coverage   66.64%   66.75%   +0.11%     
==========================================
  Files          44       43       -1     
  Lines        1826     1802      -24     
  Branches      309      304       -5     
==========================================
- Hits         1217     1203      -14     
+ Misses        483      476       -7     
+ Partials      126      123       -3

Impacted Files	Coverage Δ
src/stores/memory.ts	100% <100%> (ø)

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[kincaidoneil]

The leveldown module packaged with the connector is unused in almost all cases and causes a plethora of build errors and other issues when installing the connector

👍

Even the balance middleware is typically not used in production so on most deployments no store is being used.

FYI, I think balances were always tracked entirely in-memory... https://github.com/interledgerjs/ilp-connector/issues/478