context: add experimental code for avoiding tiny extents

context: add experimental code for avoiding tiny extents

In the current architecture we can't directly measure the physical extent
size, and we can't make good decisions with the extent data (reference)
item alone.  If the early return is enabled here, there is a small speedup
and a large drop in dedupe hit rate, especially when extent splits occur.

Leave the early return commented for now, but collect the event statistics.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>