Warmup cache for immutables signature in signer

[jpraynaud]

Issue

When a signer is setup, it is limbo for 2 epochs before being able to sign. This would be a good period of time during which the cache of the hash computation of the immutable files could be warmed up to avoid CPU spikes the first time the signer is able to sign.

Note: this feature will likely also avoid getting signatures rejected for a first time signer (because signatures are sent few minutes after the snapshot has already been certified.