Before this change we would have replaced an existing "filters" field
with the filters applied in this run. But if the user runs `onionperf
filter` more than once to apply different filters, we'll want to list
all filters in the "filters" field.

This change also removes a couple of `self.`s, in particular
`self.analysis` and `self.filters`, because one Filtering object can
apply filters to more than one analysis.