fix: use var_str in 'reject' msgs

CommandString is (de)serialized as 12 bytes. However, BIP-61 defines
the 'response-to-msg' (message that triggered the reject) field
to be a var_str [1].

[1]: https://github.com/bitcoin/bips/blob/master/bip-0061.mediawiki#common-payload

Author: 0xB10C

src/network/message.rs

@@ -472,7 +472,7 @@ mod test {
             NetworkMessage::GetCFCheckpt(GetCFCheckpt{filter_type: 17, stop_hash: hash([25u8; 32]).into()}),
             NetworkMessage::CFCheckpt(CFCheckpt{filter_type: 27, stop_hash: hash([77u8; 32]).into(), filter_headers: vec![hash([3u8; 32]).into(), hash([99u8; 32]).into()]}),
             NetworkMessage::Alert(vec![45,66,3,2,6,8,9,12,3,130]),
-            NetworkMessage::Reject(Reject{message: CommandString::try_from("Test reject").unwrap(), ccode: RejectReason::Duplicate, reason: "Cause".into(), hash: hash([255u8; 32])}),
+            NetworkMessage::Reject(Reject{message: "Test reject".into(), ccode: RejectReason::Duplicate, reason: "Cause".into(), hash: hash([255u8; 32])}),
             NetworkMessage::FeeFilter(1000),
             NetworkMessage::WtxidRelay,
             NetworkMessage::AddrV2(vec![AddrV2Message{ addr: AddrV2::Ipv4(Ipv4Addr::new(127, 0, 0, 1)), port: 0, services: ServiceFlags::NONE, time: 0 }]),

src/network/message_network.rs

@@ -26,7 +26,6 @@ use network::address::Address;
 use network::constants::{self, ServiceFlags};
 use consensus::{Encodable, Decodable, ReadExt};
 use consensus::encode;
-use network::message::CommandString;
 use hashes::sha256d;
 
 /// Some simple messages
@@ -133,7 +132,7 @@ impl Decodable for RejectReason {
 #[derive(PartialEq, Eq, Clone, Debug)]
 pub struct Reject {
     /// message type rejected
-    pub message: CommandString,
+    pub message: Cow<'static, str>,
     /// reason of rejection as code
     pub ccode: RejectReason,
     /// reason of rejectection