few cosmetics + temporary discard congestion
quick fix batch:
 1. handle a race between heca_extract_page and heca_lookup_page_in_remote
    when someone is pulling both by fault and by request.
 2. document another unlikely edge case in heca_trigger_page_pull.
 3. remove heca_swap_wrapper, as the hook is wrapper enough (it also had
    unnecessary #if defined in it).
Merge pull request #40 from roeitell/dev

Minors in push path