MINOR: filters: add per-filter call counters

MINOR: filters: add per-filter call counters

The idea here is to record how many times a filter is being called on a
stream. We're incrementing the same counter all along, regardless of the
type of event, since the purpose is essentially to detect one that might
be misbehaving. The number of calls is reported in "show sess all" next
to the filter name. It may also help detect suboptimal processing. For
example compressing 1GB shows 138k calls to the compression filter, which
is roughly two calls per buffer. Maybe we wake up with incomplete buffers
and compress less. That's left for a future analysis.