lib: Separate all phrases indexed by _notmuch_message_gen_terms

lib: Separate all phrases indexed by _notmuch_message_gen_terms

This adds a 100 termpos gap between all phrases indexed by
_notmuch_message_gen_terms.  This fixes a bug where terms from the end
of one header and the beginning of another header could match together
in a single phrase and a separate bug where term positions of
un-prefixed terms overlapped.

This fix only affects newly indexed messages.  Messages that are
already indexed won't benefit from this fix without re-indexing, but
the fix won't make things any worse for existing messages.