eth, rpc: standardize the chain sync progress counters

rpc/api/eth_js.go

@@ -42,6 +42,10 @@ web3._extend({
 		new web3._extend.Property({
 			name: 'pendingTransactions',
 			getter: 'eth_pendingTransactions'
+		}),
+		new web3._extend.Property({
+			name: 'syncing',
+			getter: 'eth_syncing'
 		})
 	]
 });