radix_tree: implement runtime_initialize_mt

which enables single-writer multi-reader concurrency. After calling
runtime_initialize_mt() each assignment (done through assing_val or
insert_or_assign) does not modify the value in-place but rather
allocates new node. Also, erase does not free nodes/leaves immediately
but adds them to a garbage list which can be processed using
garbage_collect.

Author: igchor