Better approach for prelocking of tables for stored routines execution

and some SP-related cleanups.

- We don't have separate stage for calculation of list of tables
  to be prelocked and doing implicit LOCK/UNLOCK any more.
  Instead we calculate this list at open_tables() and do implicit
  LOCK in lock_tables() (and UNLOCK in close_thread_tables()).
  Also now we support cases when same table (with same alias) is
  used several times in the same query in SP.

- Cleaned up execution of SP. Moved all common code which handles
  LEX and does preparations before statement execution or complex
  expression evaluation to auxilary sp_lex_keeper class. Now 
  all statements in SP (and corresponding instructions) that
  evaluate expression which can contain subquery have their
  own LEX.

sql/sql_class.cc

@@ -272,6 +272,7 @@ THD::THD()
     ulong tmp=sql_rnd_with_mutex();
     randominit(&rand, tmp + (ulong) &rand, tmp + (ulong) ::query_id);
   }
+  prelocked_mode= NON_PRELOCKED;
 }