hash trees: further cleanup, to make sure we're validating the right thing

hash trees: further cleanup, to make sure we're validating the right thing
hashtree.py: improve the methods available for finding out which hash nodes
 are needed. Change set_hashes() to require that every hash provided can
 be validated up to the root.
download.py: validate from the top down, including the URI-derived roothash
 in the share hash tree, and stashing the thus-validated share hash for use
 in the block hash tree.