Change default block sync size from 200 to 20

With the new sync algorithm, the network overhead will be masked
as the thread adding blocks isn't interrupted by network calls
anymore. This should reduce memory usage a lot during sync.

src/cryptonote_config.h

@@ -89,7 +89,7 @@
 
 
 #define BLOCKS_IDS_SYNCHRONIZING_DEFAULT_COUNT          10000  //by default, blocks ids count in synchronizing
-#define BLOCKS_SYNCHRONIZING_DEFAULT_COUNT              200    //by default, blocks count in blocks downloading
+#define BLOCKS_SYNCHRONIZING_DEFAULT_COUNT              20     //by default, blocks count in blocks downloading
 #define CRYPTONOTE_PROTOCOL_HOP_RELAX_COUNT             3      //value of hop, after which we use only announce of new block
 
 #define CRYPTONOTE_MEMPOOL_TX_LIVETIME                    86400 //seconds, one day