* Find the bank hash of the heaviest fork, replay if necessary.
* Make it more explicit how heaviest fork slot is selected.
* Use process_single_slot instead of process_blockstore_from_root, the latter
may re-insert banks already frozen.
* Put BlockstoreProcessError into the error message.
* Check that all existing blocks link to correct parent before replay.
* Use the default number of threads instead.
* Check whether block is full and other small fixes.
* Fix root_bank and move comments to function level.
* Remove the extra parent link check.
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