Polished entry freeing.

Polished entry freeing.

Clean Writeable entries in addition to Readables ones. Otherwise, there is no
way for the caller to completely get rid of a [large] entry the caller has
been writing! We hold the lock so this should be safe as long as slice.next
(and slice extras) are valid even in a half-baked Writeable entry.

Reset slice.next [before giving it to the caller]. As a consequence, we must
free individual slices even if cleaner is not set.