Make ledger cleanup service honor the replay progress.

[yhchiang-sol]

Problem

The current ledger cleanup service does not honor the replay progress.
As a result, if the replay progress is behind the configured --limit-ledger-size,
then ledger cleanup service might cleanup those slots that are older than
the --limit-ledger-size and make those slots unreplayable.

Summary of Changes

This PR makes ledger cleanup service honor the replay progress.
Under the hood, it allows Blockstore to maintain lowest_confirmed_slot,
which is updated upon successful confirm_slot.

A warning message will also be logged when the blockstore is not able
to keep its size under the specified --limit-ledger-size due to the
behind replay progress.

[yhchiang-sol]

Turns out that ledger_cleanup_service already honors the root that it never tries to delete anything newer than the root.
Closing the PR.