Two-tier caching / Multiple cache levels

[luser]

We'd like to be able to use a two-tier cache setup, where sccache can use one cache source as a read-only cache, and another as a read+write cache. This would be useful for two scenarios:

1. Allowing try server builds to get cache hits from the tier-3 branch buckets.
2. Allowing local developers to get cache hits from the buckets we use for CI.