Fix Erroneous Audit Info Update

[meksor]

As described by @danielhuppmann, when setting a run as the default, ixmp4 accidentally also updates that info for all other runs with the same model/scenario combination.

This happens because the set_as_default function sets all the other runs to is_default = False before setting True on the singular subject run. The query is adjusted to only update rows which are actually not already is_default = False and I implemented a way to pause the event listeners to avoid setting that field on the previous default run as well.

A repository can now pause the event listeners for a few lines like this:

with self.backend.event_handler.pause():
    # we dont want to trigger the
    # updated_at fields for this operation.
    self.session.execute(exc)
    self.session.commit()

Test is also adjusted to break if any of this ever happens again.

fixes #126

[danielhuppmann]

This is very elegant! Thanks!