Add RPC method to fetch the virtual UTXO set

[someone235]

• Implement GetVirtualUTXOSet(expectedVirtualParentHashes *DomainHash, fromOutpoint *DomainOutpoint, limit int) ([]*OutpointAndUTXOEntryPair, error) that will work like GetPruningPointUTXOs.
• Implement startP2PFlows and stopP2PFlows RPC commands.
• Add the virtual UTXO diff and the virtual parent hashes to BlockInsertionResult, and forward them as RPC notification.
• Add GetVirtualUTXOSet RPC command with similar signature, and make sure that the P2P flows are stopped when it's called.

[elichai]

This should take approximately 3 days