Currently, ReplayStage sends new roots to BlockstoreCleanupService, and
BlockstoreCleanupService decides when to clean based on advancement of
the latest root. This is totally unnecessary as the latest root is
cached by the Blockstore, and this value can simply be fetched.
This change removes the channel completely, and instead just fetches
the latest root from Blockstore directly. Moreso, some logic is added
to check the latest root less frequently, based on the set purge
interval.
All in all, we went from sending > 100 slots/min across a crossbeam
channel to reading an atomic roughly 3 times/min, while also removing
the need for an additional thread that read from the channel.
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