Create occasional snapshots while replaying

[amesgen]

Currently, when we replay from Genesis (eg when the ledger snapshot format changes, which happens from time to time), we don't create any snapshots during that period. This means that if one stops the node before the replay has finished (takes >10h on mainnet), one has to start from scratch.

The goal of this ticket is to define a simple ad-hoc criterion to use (ensuring that we don't slow down replaying due to the fact that taking a snapshot is still relatively expensive, cf #868), for example "snapshot every 30min", and create snapshots while replaying (while still keeping only the configured maximum number of snapshots).

This is

1. a quality of life improvement by making replaying "resumable", and
2. means that it isn't important for LedgerDB: implement predictable snapshotting #1575 to have special logic for creating a snapshot on startup when we replayed a lot of blocks.