[refactor] Add useful-for-dos "reason" field to CValidationState

This is a first step towards cleaning up our DoS interface - make
validation return *why* something is invalid, and let net_processing
figure out what that implies in terms of banning/disconnection/etc.

Behavior change: peers will now be banned for providing blocks
with premature coinbase spends.

Co-authored-by: Anthony Towns <aj@erisian.com.au>
                Suhas Daftuar <sdaftuar@gmail.com>

src/net_processing.cpp

@@ -961,6 +961,7 @@ void Misbehaving(NodeId pnode, int howmuch, const std::string& message) EXCLUSIV
 
 static bool TxRelayMayResultInDisconnect(const CValidationState& state)
 {
+    assert(state.GetDoS() == state.GetDoSForReason());
     return (state.GetDoS() > 0);
 }
 
@@ -975,6 +976,7 @@ static bool TxRelayMayResultInDisconnect(const CValidationState& state)
  * txs, the peer should not be punished. See BIP 152.
  */
 static bool MaybePunishNode(NodeId nodeid, const CValidationState& state, bool via_compact_block, const std::string& message = "") {
+    assert(state.GetDoS() == state.GetDoSForReason());
     int nDoS = state.GetDoS();
     if (nDoS > 0 && !via_compact_block) {
          LOCK(cs_main);