Add interrupt-block, stop-block, refactor stats, explicit validity checks

[prasannavl]

/kind feature

• Adds the followings undocumented (for now), that's useful for running automated tests to stop the chain at a certain point.
  • -interrupt-block=hash|height flag, that's used to interrupt the chain by marking the block as invalid, but leaving the node running to interact with RPC.
  • -stop-block=hash|height flag that's used to stop the node at a block height, but without invalidating the block for further runs.
• Minor refactors to rpc stats, and also ensure flags are set before the server init.
• Explicit checks on price validity checks (require full sync verification)