This code was previously misplaced so that only posts in discussion which
had not yet been processed in the current cron run would have keys
generated. If two posts were generated in the same discussion and sent out
on the same cron cycle, only the first post to be processed would have a
key generated.
These sporadic unit test failures were happening because the post time and
subscription time sometimes differ depending on the amount of time it takes
to complete the different components of the test.
To reliably address these issues, the post creation, and the discussion
subscription times are manually updated by different factors to ensure a
more realistic state of affairs.
This adds a reset of the discussion and forum cache when running the mail
and maildigest tests.
These test fails were highlighted by the addition of new tests which
add similar post ids.
The maxeditingtime needs to be a negative value for unit tests to ensure
that the messages are actually sent out.
Whilst fixing this I also discovered that the create_discussion part of the
generator also creates the first post.
