make block queue into a more generic verification queue and fix block heap size calculation (#2095)

* move block queue to own module, a couple readability changes

* make block queue generic over verifiable data

also fixes heap size calculation

* make block queue into a more generic verification queue

* some module reoganization

* implement header queue

* clean up verification error messages

ethcore/src/header.rs

@@ -295,6 +295,12 @@ impl Encodable for Header {
 	}
 }
 
+impl HeapSizeOf for Header {
+	fn heap_size_of_children(&self) -> usize {
+		self.extra_data.heap_size_of_children() + self.seal.heap_size_of_children()
+	}
+}
+
 #[cfg(test)]
 mod tests {
 	use rustc_serialize::hex::FromHex;