change prefetch thread num

[flywukong]

Description

since the main thread has become faster after using sharedStorage , we need more prefetch thread num to make the prefetch is faster than main process.

Rationale

After after adding some metric on prefetch therad , the metric shows that the prefetcher with two threads has 5-20% faster than main process while prefetcher with three threads has become 20-40% faster than main process .

The yellow line is using 3 thread

Performance

The two test machine is on AWS , the two nodes has worked in fullsync mode and has caught up to the latest block

The cost of process a block show that the processing cost of main process with three prefetch threads has become 3%-10% faster than two prefetch threads

Compare to the processing cost of main process prefetcher with 3 thread and 4 thread , the cost of process a block is nearly same , so 3 thread is enough for prefetcher

Changes

Notable changes:

• add each change in a bullet point here
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