Redundant reads and writes

[austinabell]

The issue is around the fact that a shard is persisted when a hamt is expanded here and also the cache is flushed and cleared even on read only access which leads to completely unnecessary writes.

I'll link to the PR where I add these extra writes/cache clears as I think it might be the easiest way to describe the diff and what can be improved: ChainSafe/forest#731. The functionality that matches this repo currently is under the go-interop feature and can ignore the Amt changes (I grouped this change together)

I haven't benchmarked the difference in a real use case, but generally saw the writes and reads to be between 1.5-2x more than they need to be.

[rvagg]

Eric did some further investigation and documentation work on this here too: #61

[Stebalien]

This will be fixed in #74.