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_insert.cc

@@ -433,7 +433,7 @@ abort:
   Prepare items in INSERT statement
 
   SYNOPSIS
-    mysql_prepare_update()
+    mysql_prepare_insert()
     thd			- thread handler
     table_list		- global table list
     insert_table_list	- local table list of INSERT SELECT_LEX