Minor Description
Currently whenever an arena fills up to more than 256MB, a warning is issued with a stack trace. This is very common in the case of MRS arenas, which will often be larger than 256MB.
The original intent of the warning was to catch cases where we were forgetting to Reset() an arena during a scan/compaction/etc. Given that, maybe we should be setting the "warning threshold" on a per-arena basis, or at least give the ability to suppress the warning for MRS/DMS where large arenas are the norm.