Fix stack overflow when freeing long backtrace chains

Since `RACBacktrace` forms a linked list, it's possible to thread hop _so many times_ that the resulting linked list blows the stack when freed. This autoreleases each step, similarly to how we free `RACDynamicSequence`, but less optimized.

Fixes #753 (again).

@KevinVitale Would you mind trying out this branch?