v2.0: Startup only sets latest_full_snapshot_slot if generating snapshots (backport of #3633)

[mergify]

Problem

AccountsDb uses the latest full snapshot slot to gate how much, and what, to clean/etc. The latest full snapshot slot is updated every time a new full snapshot is taken.

If snapshots are disabled, then the latest full snapshot slot is never updated, and we cannot purge zero lamport accounts past the latest full snapshot slot.

The accounts-db code is aware of this though. If latest_full_snapshot_slot is None, we can clean everything. Yay!

The problem is that we always set the latest full snapshot slot at startup to Some, thus we never clean zero lamport accounts past the startup full snapshot slot!

Summary of Changes

If we're not generating snapshots, the latest full snapshot slot should be None.

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This is an automatic backport of pull request #3633 done by Mergify.

[mergify]

Cherry-pick of 4ea10c2 has failed:

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Unmerged paths:
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	both modified:   accounts-db/src/accounts_db.rs
	both modified:   ledger/src/bank_forks_utils.rs

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[jeffwashington]

lgtm

[HaoranYi]

lgtm

[t-nelson]

this comment implies that there is yet more implicit logic in this area? can we just eliminate it?

[brooksprumo]

can we just eliminate it?

What is the "it" here?