Pretagovc

Bug fixes:
- Scheduler filter to send only to a channel option.
- Made the last_trigger for the timed scheduler only record when there are newsletters sent.

Tests are passed except:
- queue.txt (believe it is a test from zc.queue (https://github.com/zopefoundation/zc.queue/blob/master/src/zc/queue/queue.txt)
- ValueError: _p_serial must be an 8-character bytes array, due to somehow the new 'persistent' module is being used, in spite of the fact that ZODB 3.10.x bundles its own version. The separately-released version is strict about non-bytes values as OIDs / TIDs. 
- missing module 'plone.testing' and 'zope.app.pagetemplate' during test

I don't have good knowleadge to make all these tests passed again.