ObjFW  Check-in [e7f4f80e23]

Getting a pointer and increasing it until we reach the top pointer does
not work: Releasing an object can temporarily create new autorelease
pools, which can trigger resizing of "objects" using realloc, which can
move it to a different address, which will then lead to continuing to
iterate on a now invalid pointer.

This is now solved by using an index into "objects" instead. Since we're
now indexing for the pop, let's use indexes everywhere, as they're more
readable anyway.

While debugging this, I noticed that the last pool is popped quite
frequently, only for a new pool to be pushed immediately again. This
resulted in a free followed by a malloc every time. Instead, keep the
pool, but let OFThread explicitly say when to free everything.