The new struct tree_iterator and associated start/get/next/free
functions replace the recursive walk() function, removing the need for a
callback when iterating over all nodes in the tree, and allowing
iteration to be suspended while other pseudo-threads are run. This
allows an HTTP REST request to keep a tree_iterator in its state struct
and potentially simplifies other areas of the code.
The iterator free()s any empty internal tree nodes that it encounters,
as did the original tree_walk() function. To support the existence of
multiple iterators at once, a reference count has been added to the
tree_node struct, to prevent any iterator from free()ing a node while
any other iterators point to it; only the last iterator to pop out of an
empty node will free() it.
The tree_walk() and tree_walk_prefix() functions have been
re-implemented to use an iterator state object internally. This
resolves an outstanding TODO to perform tree-node freeing during a
prefix walk, and simplifies the code considerably.
Renamed some function parameters and struct members to make the
nibble-tree API a little more self-explanatory.
Added a nibble-tree test to the 'serval-tests' utility.
