We wait a while after deciding a huge extent should get hugified to see if it
gets purged before long. This avoids hugifying extents that might shortly get
dehugified for purging.
Rename and use the hpa_dehugification_threshold option support code for this,
since it's now ignored.
Currently that just means max_alloc, but we're about to add more. While we're
touching these lines anyways, tweak things to be more in line with testing.
