Replace banned.dat with table in cache.db

[hewison-chris]

Before cache.db existed a binary file named banned.dat was created to persist the list of nodes that have failed to respond to a number of requests. It was later updated to also include the validators which are whitelisted so that they are not banned. To make it easier to maintain and query the current state of banned and whitelisted nodes, we should store this in the cache.db.
As a side effect this should also fix an issue we are seeing in our PROD environment where nodes are not able to fetch unknown txs. (#2603)