[Trinity Reverify]: Better memory throttling mechanism needed for Query

We currently have memory-quota per query to kill runaway queries. But that's just the 1st step.

If there are multiple concurrent queries (each inheriting the same memory-quota), they can still bring down the system cumulatively and none of them would be considered a runaway if within the limit.

To avoid situations like those, we need a system-wide query service memory-quota (across multiple queries), especially for the cloud, where many users could use just the primary index (and not have well-defined indexes).