fix: simplify preloader logic

[rvagg]

After writing a doc commit into @hannahhoward's #277 to clarify the preloader Loader() flow, I noticed a possible wasteful write @

lassie/pkg/retriever/bitswaphelpers/preloadcachingstorage.go

Lines 311 to 316 in 2b3656b

	// write to parent and cache and return
	return loadTo(
	r,
	[]linking.BlockWriteOpener{cs.cacheLinkSystem.StorageWriteOpener, cs.parentLinkSystem.StorageWriteOpener},
	linkCtx,
	link)

At that point, the Loader() call has failed to find the block in the parent or cache linksystems and it's not in the preload queue, so it's a new block that hasn't been encountered before and we load it from the "Fetcher" (bitswap) directly, bypassing any messy preload semantics. But after we have it, we write it both to the cache and to the parent. But, once it's in the parent, we shouldn't need it in the cache anymore. It's up to the user of the preloader to ensure that the parent linksystem is also able to cache blocks for duplicate-block traversals, and it already does for us—we have a messy 3-way CAR + Stream output attached to this as both the parent and cache, so the double write is entirely wasteful.

That then exposes some other simplifications for the code, which is very nice. 17 additions and 40 deletions.

[codecov-commenter]

Codecov Report

Merging #284 (4ef872b) into fix/missing-block-multiple-protocols (2b3656b) will decrease coverage by 1.50%.
The diff coverage is 73.33%.

Additional details and impacted files

@@                           Coverage Diff                            @@
##           fix/missing-block-multiple-protocols     #284      +/-   ##
========================================================================
- Coverage                                 73.07%   71.57%   -1.50%     
========================================================================
  Files                                        70       69       -1     
  Lines                                      6651     6068     -583     
========================================================================
- Hits                                       4860     4343     -517     
+ Misses                                     1547     1486      -61     
+ Partials                                    244      239       -5     

Impacted Files	Coverage Δ
.../retriever/bitswaphelpers/preloadcachingstorage.go	78.45% <73.33%> (-1.55%)	⬇️

... and 19 files with indirect coverage changes

[hannahhoward]

LGTM