Revert the coins cache leveldb block_size back to 4096
Revert the coins cache leveldb block_size back to 4096

This is the original default value and gives us a much
better cache hit ratio when we startup the node
after having interrupted IBD.  As a result startup time
is much faster, particularly when running a node on hard disk.
For example, starting up on scalenet with 128MB blocks takes only
1 to 2 minutes, wherease without these changes it was taking 15minutes.
Also, disk activity is reduced and the disk is no longer pegged at 100%.

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