Rewrite message parsing using string_view

It's a bit too early yet to require C++17 so the implementation from
BackportCpp (string_view-standalone) is used instead.

Fixes https://crbug.com/oss-fuzz/34413 - slow message parsing on huge
messages. In real word, messages can't be that big, because CSocket
enforces a line length limit.

This can be considered a regression of 1.7.0, because before it, instead
of gathering params into a vector, code was searching 1st word in the
string, then 2nd word, then 3rd word, starting from beginning each time.
It was not very efficient, but the number of passes over the string was
limited.

include/znc/Message.h

@@ -161,7 +161,7 @@ class CMessage {
     };
 
     CString ToString(unsigned int uFlags = IncludeAll) const;
-    void Parse(CString sMessage);
+    void Parse(const CString& sMessage);
 
 // Implicit and explicit conversion to a subclass reference.
 #ifndef SWIG