Support for character set conversion in binary protocol: another go

after Monty's review.
- Item_param was rewritten.
- it turns out that we can't convert string data to character set of
  connection on the fly, because they first should be written to the binary
  log.
  To support efficient conversion we need to rewrite prepared statements
  binlogging code first.

sql/sql_string.h

@@ -299,4 +299,7 @@ public:
     return FALSE;
   }
   void print(String *print);
+
+  /* Swap two string objects. Efficient way to exchange data without memcpy. */
+  void swap(String &s);
 };