[coordinator] Add high concurrent carbon/graphite downsampling integration test

[robskillington]

What this PR does / why we need it:

This adds high concurrent downsampling tests for Carbon/Graphite ingestion. This catches issues that would occur due to flush concurrency and load.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

NONE

Does this PR require updating code package or user-facing documentation?:

NONE