Check total memory limit when allocating user memory

[rschlussel]

This prevents queries from exceeding the total memory limit between the
time that user memory is allocated, and the next system memory
allocation.

Test plan - unit test

== RELEASE NOTES ==

General Changes
* Enforce ``max_total_memory_per_node`` on user memory allocation

[pettyjamesm]

Makes sense as a matter of closing the gaps of the current implementation. I do wish this code wasn’t so fragile in regard to potential deadlocks and lock contention or that it didn’t use a “read then update” pattern on the memory pool to avoid it- but that’s definitely a larger refactor for another day.

[rschlussel]

Makes sense as a matter of closing the gaps of the current implementation. I do wish this code wasn’t so fragile in regard to potential deadlocks and lock contention or that it didn’t use a “read then update” pattern on the memory pool to avoid it- but that’s definitely a larger refactor for another day.

Agreed 100%. I don't have bandwidth for it, but memory management could do with a good refactoring.

[rschlussel]

ooh looks like all the testMemoryReservationYield tests relied on the old behavior. I'll update those.

[rschlussel]

tests pass now. Merging.