Time based snapshotting

[cjen1-msft]

Snapshots are currently triggered only on transaction count (snapshots.tx_count in the configuration). It would be convenient to add a delay option too, symmetric to what we have for ledger_signature (i.e. whichever comes first).

[ivarprudnikov]

It would especially be useful when then ledgers do not receive enough transactions so that the limit of any chunk_size or tx_count is reached and there is a need to share up to date (committed) ledger files for auditing.

The alternative to force this is to send a proposal to trigger_ledger_chunk documented in https://microsoft.github.io/CCF/main/operations/ledger_snapshot.html

[achamayou]

The flip side if the timeout is low is that this will result in a lot of files, and slow down historical queries as a result. It's probably worth testing on the actual storage that's being used in production settings to find the right balance.