Make sort_deps more efficient, and also only alter order when necessary.  This means by default we run in serial number order, only going outside when we detect a dependency.
Maintain trace file order in original sort, so sort_deps doesn't mess it up.

We still need serial numbers: sort_deps can have multiple solutions for a single key, but these may deadlock with the ordering requirements of other keys.  By sticking close to the actual order (ie. serial order), we minimize the chance of this happening.

In my tdbtorture -n 4 example trace, this reduces from 14493 to 3210 dependencies, but doesn't make any measurable improvement in the time.  Still, it's simple to do and might make a difference for larger sets.

Also, using a single structure makes talloc_free convenient for when we do optimization.

Still deadlocks in one case, due to spurious dependencies inside traversals.  See next commit.

realloc() has that horrible overloaded free semantics.

By faking them out to length 1, we might go oob.  Just fake the malloc.

Important change:  array_pop no longer checks if the array has elements; use array_pop_check in its place.