Implement Routing Cache

[monoman]

Needs to support:

• Seeders List (routes in this list never expire)
• Blacklist (routes here never are cached)
• Whitelist (routes here priority never expire but may be purged from cache in extreme conditions)
• Cache persistence (at least for routes in Seeders/White Lists)

Hard Questions:

• How to abstract routes, and peer promotion, to support any protocol?
• How to expose the cache to the NodeSink?

Some basics for a memory-based implementation, already in the repo, but: no expiration, no persistence, no reversible connections, yet

[monoman]

Some work already done on this, but without persistence or LRU purging.