Fix potential chunk leaks.

Move chunk_dalloc_arena()'s implementation into chunk_dalloc_wrapper(),
so that if the dalloc hook fails, proper decommit/purge/retain cascading
occurs.  This fixes three potential chunk leaks on OOM paths, one during
dss-based chunk allocation, one during chunk header commit (currently
relevant only on Windows), and one during rtree write (e.g. if rtree
node allocation fails).

Merge chunk_purge_arena() into chunk_purge_default() (refactor, no
change to functionality).