Fail queries early when their minimum reservation is too high to execute within the given mem_limit

With the initial version of IMPALA-3200, we won't check whether the minimum reservation fits within the query memory limit until fragment startup time.

There are a couple of decisions to make here:

• How to deal with the RESERVATION_MEM_FRACTION/RESERVATION_MEM_MIN_REMAINING heuristic that limits reservation relative to the query mem_limit
• Where to actually reject the query

This is a bit tricky since it needs to happen after we've determined the applicable memory limit (which depends on resolving the request pool). Admission control would be one place but admission control can be disabled.