a4/a4 — pull request #193 by pwaller (2012-11-06 09:43:40+00:00)
Title: Split input_stream_impl into abstract base class and input_stream_a4_impl

This pull request depends on #192. (It contains some of the same commits).

The purpose is to introduce new forms of `InputStreamImpl` which can provide
input messages, for example, a processor's output stream or a ROOT input stream.