net: Add and document network messages in protocol.h

- Avoids string typos (by making the compiler check)
- Makes it easier to grep for handling/generation of a certain message type
- Refer directly to documentation by following the symbol in IDE
- Move list of valid message types to protocol.cpp:
    protocol.cpp is a more appropriate place for this, and having
    the array there makes it easier to keep things consistent.

src/alert.cpp

@@ -138,7 +138,7 @@ bool CAlert::RelayTo(CNode* pnode) const
             AppliesToMe() ||
             GetAdjustedTime() < nRelayUntil)
         {
-            pnode->PushMessage("alert", *this);
+            pnode->PushMessage(NetMsgType::ALERT, *this);
             return true;
         }
     }