Implement configurable fast node cache size

[p0mvn]

Context

There is a config iavl-cache-size that applies to both regular nodes and fast nodes. We would like to introduce a new config parameter specifically for fast nodes. Additionally, we would like to represent both parameters in bytes as opposed to the number of nodes.

Currently ,iavl-cache-size config is applied to every iavl substore (each module has its own substore). As a result, for every sub store we have 2 caches of the same max size - one for regular nodes and one for fast nodes.

While it might make sense to keep the same value for regular node, the current value is simply too large for fast nodes. As a result, we keep indefinitely populating fast node cache until OOM. That's why this separation is desired.

Some additional context here: #1140 (comment)

Currently, we hardcoded fast node cache to 100K: osmosis-labs/iavl#37

Plan Ahead

• Refactor fastNodeCache and nodeCache to be separate modules with synchrony iavl#24
• refactor iavl cache max represented in terms of both bytes and the number of nodes #1187
• investigate if it makes sense to replace custom LRU cache implementation with one of:
  • https://github.com/golang/groupcache/blob/master/lru/lru.go
  • https://github.com/hashicorp/golang-lru
• let fast node cache use the bytes version, find the most optimal value, and test
• potentially let regular nodes also use the bytes implementation and remove the other one

Work in Progress

• implement cache abstraction and unit test iavl#38
• add byte limit implementation for cache iavl#40

Acceptance Criteria

• implement a separate configurable value for fast nodes cache called iavl-fast-cache-size
• represent both iavl-fast-cache-size and iavl-cache-size in bytes
• find the best parameter for both that does not cause OOM while maintaining efficient performance

[ValarDragon]

Lets move this to IAVL if we still want to pursue this!