Inner-tree thread safety implemented

See the updated README.md for details.

Also some minor bugs identified and fixed:
- qnode_remove() wasn't decrementing q->cnt (oops!)
- qtree_setMaxNodeCnt() didn't check for 0 values, which would have
  caused insertions to inf-loop in nasty ways. 1 is now the minimum,
  if a 0 is passed.