clear.
cache_eviction_maximum_page_size: collection statistic, aggregate
doesn't apply, don't clear.
various txn_checkpoint_XXX statistics: collection statistic, aggregate
doesn't apply
various Btree maximum size statistics: aggregate the maximum value,
rather than doing no aggregation at all.
lsm_chunk_count: aggregate the count of LSM chunks.
Add syntax checking code to stat.py to disallow aggregation flags
for connection-level statistics.
Reference #1742.
It is often useful to know if a cache is full of internal pages. I
could have added page count tracking, but byte count tracking seems
more useful (enough to justify the extra performance overhead).
