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.

mysql-test/r/lock.result

@@ -42,7 +42,7 @@ check table t2;
 Table	Op	Msg_type	Msg_text
 test.t2	check	error	Table 't2' was not locked with LOCK TABLES
 insert into t1 select index1,nr from t1;
-ERROR 42000: INSERT command denied to user 'root'@'localhost' for column 'index1' in table 't1'
+ERROR HY000: Table 't1' was not locked with LOCK TABLES
 unlock tables;
 lock tables t1 write, t1 as t1_alias read;
 insert into t1 select index1,nr from t1 as t1_alias;