Warm up the SRS and circuit key caches in the end to end tests

[jpraynaud]

Why

The end to end tests do not prepare the SNARK caches. Each node downloads the SRS and computes its circuit keys lazily during the first aggregation, which forces a long epoch length and makes the test runs slow and vulnerable to a download timeout.

What

Prepare the SRS and the circuit keys before the nodes start, so the first aggregation only pays for the proving.

How

• Add a cache warmup step to the end to end infrastructure setup (mithril-end-to-end/src/mithril/infrastructure.rs)
• Share a single cache directory across the nodes of a run, or seed each node from a prepared one
• Reduce the epoch length once the warmup removes the initial cost
• Report the warmup duration separately from the test duration
• How to deal with cache invalidation (e.g. when the circuit verification keys change)?