operator-- may fail in concurrent radix_tree

[lukaszstolarczuk]

operator-- may fail (when MtMode is enabled) in some specific cases, e.g.:

• try_decrement fails due to concurrent erase (it can be the element pointed to by the iterator)
• operator-- calls lower_bound which returns end() (the tree is now empty)
• try_decrement crashes because we decrement beyond begin()

ref. #1156

[igchor]

There are two problems for this version: 6300980

1. If iterator points to end() and try_decrement fails, we will call key() on nullptr (leaf_ == nullptr for end())
2. Problem mentioned by lukaszstolarczuk is caused by a lack of special value for begin(). When we use operator++ and encounter end() we know it's time to stop. For operator-- be cannot rely on comparison with begin() since its value can change. We could introduce a special past-begin iterator for this purpose.