improve minimum difficulty handling (#1791)

* rename MIN_DIFFICULTY to UNIT_DIFFICULTY; define Difficulty::unit, fix INITIAL_DIFFICULTY
* improve minimum difficulty handling
* replace all Difficulty::one by ::min
* revert secondary scaling default in HeaderInfo; rename scaling_difficulty; refactor difficulty_data_to_vector

api/src/types.rs

@@ -509,8 +509,8 @@ pub struct BlockHeaderPrintable {
 	pub cuckoo_solution: Vec<u64>,
 	/// Total accumulated difficulty since genesis block
 	pub total_difficulty: u64,
-	/// Difficulty scaling factor between the different proofs of work
-	pub scaling_difficulty: u32,
+	/// Variable difficulty scaling factor for secondary proof of work
+	pub secondary_scaling: u32,
 	/// Total kernel offset since genesis block
 	pub total_kernel_offset: String,
 }
@@ -531,7 +531,7 @@ impl BlockHeaderPrintable {
 			edge_bits: h.pow.edge_bits(),
 			cuckoo_solution: h.pow.proof.nonces.clone(),
 			total_difficulty: h.pow.total_difficulty.to_num(),
-			scaling_difficulty: h.pow.scaling_difficulty,
+			secondary_scaling: h.pow.secondary_scaling,
 			total_kernel_offset: h.total_kernel_offset.to_hex(),
 		}
 	}