note if big key or data in output
The page memory footprint is increased when large keys are brought into
memory, which can overflow 32 bits.  Make WT_PAGE.memory_footprint 64B,
minor shuffling of the WT_PAGE layout to avoid growing it.
Move the overflow tracking discard functionality from the general-
purpose discard functions into the overflow tracking sources, make
it self-contained.
Break large reads/writes into 1GB chunks (FreeBSD has an INT_MAX limit
on pread/pwrite calls).
The returned size is a (size_t), not a (uint32_t).
Remove WT_STORE_SIZE() cast of the size when incrementing/decrementing
WT_PAGE.memory_footprint; size may overflow 4B, and memory_footprint is
now an 8B type.
Change the output line when larger than a GB value, so it's clear when
we're going for the gold.