Generating MISP feed on every incoming message slows down processing.
The new config option let us decide to save them in batches. Cached
events are stored in a cache list in Redis. In addition, a code
related to Python 3.6 was removed as we do not support this version
any more.

The bot can now construct an event much more alligned to custom
needs, allowing setting comments and selecting just a subset of
fields to export

With `event_separator` parameter, user can decide to create
more than one MISP event in the output bot and group incomming
messages based on given field.

In additon, the message library was fixed not to modify the
parameter directly.

A lot of tests depend on that, so it looks currently risky
to change.