Apply new difficulty algo, remove configurable cuckoo size

Integrate the new difficulty calculation into the block chain
validation, the miner and tests. As the difficulty calculation
doesn't use varying Cuckoo sizes anymore and we narrowed down
reasonable final Cuckoo Cycle parameters, removed all Cuckoo
Cycle sizes from block headers.

Formalized easier Cuckoo Cycle sizes for testing (and possibly
testnet) by introducing a test mode in configuration. Updated
all tests.

src/bin/grin.rs

@@ -282,7 +282,7 @@ fn read_config() -> grin::ServerConfig {
 
 fn default_config() -> grin::ServerConfig {
 	grin::ServerConfig {
-		cuckoo_size: 12,
+    test_mode: true,
 		seeding_type: grin::Seeding::WebStatic,
 		..Default::default()
 	}