Resource group may have memory overuse in below case:

	CREATE RESOURCE GROUP rg_concurrency_test WITH
	(concurrency=1, cpu_rate_limit=20, memory_limit=60,
	 memory_shared_quota=0, memory_spill_ratio=10);
	CREATE ROLE role_concurrency_test RESOURCE GROUP rg_concurrency_test;

	11:SET ROLE role_concurrency_test;
	11:BEGIN;

	21:SET ROLE role_concurrency_test;
	22:SET ROLE role_concurrency_test;
	21&:BEGIN;
	22&:BEGIN;

	ALTER RESOURCE GROUP rg_concurrency_test SET CONCURRENCY 2;

	11:END;

The cause is that we didn't check overall memory quota usage in the
past, so pending queries can be waken up as long as the concurrency
limit is not reached, in such a case if the currently running tranctions
have used all the memory quota in the resource group then the overall
memory usage will be exceeded.

To fix this issue we now checks both concurrency limit and memory quota
usage to decide whether to wake up pending queries.
Signed-off-by: NZhenghua Lyu <zlv@pivotal.io>