radix_tree: api change

radix_tree: api change

Instead of using 'key state & key end' uint8_t* switch to use
void* key, & size_t keylen.  This allows easier adaptation with
lvm code base with way too much casting with every use of function.

Also correctly mark const buffers to avoid compiled warnings and
casting.

Adapt the only bcache user ATM for API change.

Adapt unit test to match changed API.