When bounces_drop_all=true, filters such as receive_encrypted_only were
circumvented and allowed non-encrypted senders to pass messages onto the
lists (see #508).

This moves the bounce-dropping mechanism from `filters_runner.rb` to
`runner.rb`. Thus, the decision if a bounce is being dropped will be made
entirely in runner.rb. `filters_runner.rb` will only take care of checking
for errors.