LedgerDB: implement predictable snapshotting

[amesgen]

Closes #1424

Based on top of #1513, see there for the relation to this change.

Note that we need #1573 before this can be released in a node.

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Currently, this PR replaces the previous logic for when to create snapshots (It would be possible to preserve it, but I don't see a big motivation). Concretely, SnapshotFrequencyArgs contains

• sfaInterval :: SlotNo: Create snapshots every sfaInterval many slots.

  Default: 2*k = 4320 slots, so 72 min on mainnet as before.

• sfaOffset :: SlotNo: Allows to determine the offset of where snapshots are taken, see below.

  Default: 0

• sfaRateLimit :: DiffTime: A minimum duration between snapshots (used to avoid excessive snapshots while syncing).

  Default: 10 minutes (previous value was 6 minutes, which seemed a bit low, so I increased it somewhat)

Concretely, the node will try to create snapshots for the last immutable blocks before the slots

sfaOffset, sfaOffset + sfaInterval, sfaOffset + 2 * sfaInterval, sfaOffset + 3 * sfaInterval, ...

but can skip creating some of these depending on the sfaRateLimit (which can be disabled by setting it to a non-positive value). Also see the Haddocks.

For example, setting sfaInterval = 10*2160*20 (one mainnet epoch) and sfaOffset = 0 will cause the node to create snapshots for the last block in every Shelley epoch. By setting sfaOffset to eg 5*2160*20 (half an epoch), snapshots are created just before the midway point in each epoch.

There is some code that could be shared between V1 and V2 (already even before this PR), but given the upcoming removal of V1, this seems acceptable for now.

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TODO: Tests

[amesgen]

I integrated this into 10.5 for testing:

• Consensus branch: amesgen/predictable-snapshots-10.5

• Node branch: amesgen/test-snapshot-stuff

  Currently, this treats SnapshotInterval (which previously was a number of seconds) as a number of slots. This is convenient, and should be fine for mainnet AFAICT, but maybe we still want to add some kind of warning (see also the TODO in the code there).

[jasagredo]

Could we provide an example relating this with the intended use on an epoch? Using some real values.

[amesgen]

I added the same example as in the PR description 👍

[jasagredo]

Suggested change

	-- number @slot@.
	-- number @slot@ or younger.

?

[amesgen]

Each s in snapshotSlots is the slot of a ledger state in DbChangelog (see the contract of onDiskSnapshotSelector), so it is true as written. (Otherwise, we wouldn't take a snapshot for the requested slot, so snapshots wouldn't be predictable.)