caches: introduce a function to warm cache We have multiple caches that gain from being kept up to date. For example in a server setup, we want to make sure the branchcache cache is hot for other read-only clients. Right now each cache tries to update themself in place where new data have been added. However the approach is error prone (we might miss some spot) and fragile. When nested transaction are involved, such cache updates might happen before a top level transaction is committed. Writing caches for uncommitted data on disk. Having a single entry point, run at the end of each successful transaction, helps to ensure the cache is up to date and refreshed at the right time. We start with updating the branchmap cache but other will come.