Secondary PoW scaling factor dampening, cleanup (#1765)

* Remove useless time median window
* Secondary PoW factor dampening
* Fix off-by-one in time window, cleanup dampening, fix tests

servers/src/grin/server.rs

@@ -400,7 +400,6 @@ impl Server {
 			let last_blocks: Vec<consensus::HeaderInfo> =
 				global::difficulty_data_to_vector(self.chain.difficulty_iter())
 					.into_iter()
-					.skip(consensus::MEDIAN_TIME_WINDOW as usize)
 					.take(consensus::DIFFICULTY_ADJUST_WINDOW as usize)
 					.collect();