Improve Lightnodes to use Phi, support caching requests and more

[jazg]

Lightnodes have been updated to use the Phi framework. This update involves numerous improvements such as:

• Basic request validation
• TTL and LRU caching
• Background polling for nodes that have joined or left the network
• Conditional dispatching of requests based on the method (send to leader or n random nodes)
• Conditional response consolidation based on the method (return first or majority responses)
• Ability to send requests to specific Darknodes (?id=8M...)