Alert system DoS prevention

This fixes two alert system vulnerabilities found by
Sergio Lerner; you could send peers unlimited numbers
of invalid alert message to try to either fill up their
debug.log with messages and/or keep their CPU busy
checking signatures.

Fixed by disconnecting/banning peers if they send 10 or more
bad (invalid/expired/cancelled) alerts.

src/main.h

@@ -1535,7 +1535,7 @@ public:
 
     uint256 GetHash() const
     {
-        return SerializeHash(*this);
+        return Hash(this->vchMsg.begin(), this->vchMsg.end());
     }
 
     bool IsInEffect() const