On some slow host, the `newsletter` table get filled with
duplicated `notification_history` marked as `sending` that
never get sent.
To prevent this we've made the two following changes:
* We prevent firing `publis_*` hooks on post_type which
are excluded from search.
* We do not schedule a new post notification email if one
has an `notification_history` entry marked as `sending`.
[MAILPOET-1371]
