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_trigger.h

@@ -42,14 +42,21 @@ public:
 
     if (bodies[event][time_type])
     {
-      /*
-        Similar to function invocation we don't need to surpress sending of
-        ok packets here because don't allow execute statements from trigger.
+#ifndef EMBEDDED_LIBRARY
+      /* Surpress OK packets in case if we will execute statements */
+      my_bool nsok= thd->net.no_send_ok;
+      thd->net.no_send_ok= TRUE;
+#endif
 
+      /*
         FIXME: We should juggle with security context here (because trigger
         should be invoked with creator rights).
       */
       res= bodies[event][time_type]->execute_function(thd, 0, 0, 0);
+
+#ifndef EMBEDDED_LIBRARY
+      thd->net.no_send_ok= nsok;
+#endif
     }
 
     return res;