Add a case to channel_filter_benchmark which uses a final resampling stage.
Use rational_resampler_ccf instead of arb_resampler_ccf.

Bug <http://gnuradio.org/redmine/issues/713> causes reuse of a
pfb_arb_resampler to hang the flowgraph. Therefore, use
rational_resampler instead in MultistageChannelFilter. In order to do
this efficiently, we need to do a bit more math to arrange a suitable
decimation and interpolation.

Also add resampler stage info to .explain().