hashtree_tree: better cleanup before creating node

extends hashtree:destroy to take a path and pass it directly to
eleveldb
prevent leakage of hsahtree_tree snaphots

delete the ets data if its still valid, catch the error otherwise
change metadata manager/hashtree to use infinity timeouts
link hashtree update proc to metadata_hashtree

otherwise the caller is not notified of its crash and waits
infinitely
add comments to metadata iterator functions
use sha for metadata object hash instead of phash2
cleanup metadata exchange logging