A cleanup for MCOL-4064 Make JOIN collation aware

A non-JOIN condition like `WHERE c1=c2` (with c1 and c2 being columns of the
same table) was not collation-aware yet after the main patches for MCOL-4064.

Additionally fixing StrFilterCmd::compare*() to address this.

utils/common/conststring.h

@@ -31,6 +31,9 @@ public:
   ConstString(const char *str, size_t length)
       :mStr(str), mLength(length)
   { }
+  explicit ConstString(const std::string &str)
+      :mStr(str.data()), mLength(str.length())
+  { }
   const char *str() const { return mStr; }
   size_t length() const { return mLength; }
   ConstString & rtrimZero()