nodestack: bugfix - rewrite 'may-read-from' and 'future values' as the same set

There were some subtle bugs that appeared because of the difficulty in
iterating over two separate sets that represent the "read from" set for
a particular read. Now, we expose an interface where there's just a
single pair of functions 'increment_read_from()' and
'read_from_empty()', for iterating over this set, along with a
flag-check 'get_read_from_status()', so that we know where to grab
values from (get_read_from_past() or get_future_value()).

This also does a lot of code moving and documentation in nodestack.cc,
for better readability and organization.