The new-style MAX_EXECUTION_TIME hint is just a new interface for the
old-style MAX_STATEMENT_TIME hint, however, there were some unexpected
divergences in the implementation:

1. The MAX_STATEMENT_TIME clause is allowed for top-level SELECT
statements only, but MAX_EXECUTION_TIME was allowed in subqueries as
well.

2. The MAX_STATEMENT_TIME clause is not allowed in SELECT statements
within stored programs (stored procedures, stored functions, triggers,
events), but MAX_EXECUTION_TIME was allowed in SPs.

The fix updates the MAX_EXECUTION_TIME implementation to ignore
that hint in subqueries/withing SP/trigger/event bodies with a
warning.