[no-release-notes] go/store/nbs: During a GC process, take dependencies on chunks that are read through the ChunkStore.

[reltuk]

Previously GC was constructed to walk the transitive closure of reachable chunks from when it started, and to take additional dependencies on any chunks that were written to the ChunkStore during the GC process. This change makes it so that we can additionally take dependencies on any chunks that are read from the ChunkStore during the GC process.

For the nbs layer to signal a new dependency during a GC, it makes use of a keeperFunc, which is passed into the BeginGC call. The contract is that, if keeperFunc is non-nil and it returns false, the chunk address which was given to the function will make its way back to a call to SaveHashes on some MarkAndSweeper whose contents will end up in the final store once the GC is completed. keeperFunc, however, is allowed to return true, which signals to the block store that the in-progress GC has passed a certain point in time and can no longer make that guarantee. As a result, anytime keeperFunc returns true, the block store implementation needs to block until the GC process is over, and then it should resume the operation from the beginning.

That machinery already exists for the Put() and Commit() use case. This PR adds machinery to do all of that on various read paths that explicitly touch chunks, such as Has, HasMany, Get, GetMany and GetManyCompressed. In order to add dependency tracking on the read path, it's important for the GC process itself to not cause the chunks it has read to form additional dependencies. This PR makes a slight change to markAndSweeper so that it can fetch chunks from the source store without recording unnecessary duplicate dependencies and potentially becoming mired in deadlocks.

[coffeegoddd]

@reltuk DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
e423a5c	ok	5937457

version	total_tests
e423a5c	5937457

correctness_percentage
100.0

[coffeegoddd]

@reltuk DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
6ad9b37	ok	5937457

version	total_tests
6ad9b37	5937457

correctness_percentage
100.0