Fix SlotMeta connected tracking (#28069)

Fix SlotMeta is_connected tracking

The tracking of connected status was previously based upon an assumption
that would be practically false for all validators. The connected status
of slots played into whether Blockstore would signal ReplayStage that it
had new shreds ready to be replayed. Prior to the change, we would never
signal and ReplayStage would always wait the entire duration of a 100ms
timeout before restarting its' main processing loop.

This commit introduces a change where we mark snapshot slots as
connected. A validator may not have a path all the way back to genesis
itself; however, snapshots are taken at known roots so we extend the
connected status to these slots. Once a node has been bootstrapped once
to have is connected, the logic persists in Blockstore such that all
children on the main fork also get their connected status updated
properly.