Fix and refactor runs_dirty-based purging.

Fix runs_dirty-based purging to also purge dirty pages in the spare
chunk.

Refactor runs_dirty manipulation into arena_dirty_{insert,remove}(), and
move the arena->ndirty accounting into those functions.

Remove the u.ql_link field from arena_chunk_map_t, and get rid of the
enclosing union for u.rb_link, since only rb_link remains.

Remove the ndirty field from arena_chunk_t.