feat: cache eviction strategy

[jgoux]

We need a strategy to delete unused / old cached databases. We also need to delete cached databases if our ephemeral storage reach a threshold.

The strategy works on two metrics:

• Disk usage: if the disk is 90% full, then we delete cached databases in old->newer order until we are back under the threshold
• TTL: we delete the cached databases from the cache if they're older than 24 hours

We execute this strategy on every requests, but we save the last execution time to a file so the strategy is only applied once per hour.