The improvements in the range-export code lead to avoid reading most blocks
twice, as well as to allowing some blocks to be written to disk multiple times.
The cache hit-rate went down from being close to 50% to a maximum of 12% at
the very end of the export. The reason is that most CIDs are never read twice
since they are correctly tracked in the CID set.
These numbers do not support the maintenance of the CachingBlockstore
code. Additional testing shows that removing it has similar memory-usage
behaviour and about 5 minute-faster execution (around 10%).
Less code to maintain and less options to mess up with.
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