The classes using Status.as_home_timeline, namely Feed and
PrecomputeFeedService are expected to filter direct statuses as
FanOutWriteService does, but their filtering were incomplete or missing.

This commit solves the problem by filtering direct statuses in
as_home_timeline as the other similar methods such as as_public_timeline
does.

* Fix #2619 - When redis feed is empty, fall back to database

* Use redis value to return feed from database only while RegenerationWorker
hasn't finished running

* Fix specs

* Replace usage of reject!

* Add spec for precompute feed service

* Refactor PrecomputeFeedService

* spec wip

make regeneration worker unique, (only schedule/execute once at a time)

as well as formatting of content

and atom feed generation to some extent, as well as the way mentions are
stored