Async snapshot generation and evidence

[jumaffre]

Resolves #1301

We now generate snapshots asynchronously on the primary, so that the consensus thread is not immediately blocked when a snapshot needs to be generated (note that with 0 worker thread, the snapshot is still generated/serialised on the consensus thread). The evidence of the snapshot (i.e. hash of serialised snapshot) is now also stored in a new ccf.snapshot_evidences table.

• I've also separated the snapshot generation (cheap) from its serialisation (expensive) in the KV store. The snapshot is taken by the consensus thread at the time we snapshot but the serialisation only happens later, possibly on a different thread.
• If there are multiple worker threads, we round-robin the generation of snapshots.

[ghost]

async_snapshot_generation@11756 aka 20200819.12 vs master ewma over 50 builds from 11354 to 11749