PROTON-772: New proton logging code

[Andrew Stitcher <as...@redhat.com>]

Alan recently landed a change which stops the proton library from
writing directly to stderr/stdout - this is a good thing (IMO).

However I question a couple of things:

1. This change adds to the external API and so should have been reviewed
(again IMO). reviews.apache.org now does work with the proton git repo.

2. I don't think this change should be an external API at all - at least
not initially, perhaps with some user pressure.

3. There is already logging code triggered by PB_TRACE_FRM,
PN_TRACE_RAW, PN_TRACE_DRV it would make sense for this to be integrated
with the new work. Especially as this allows you to specify the tracer
used (using pn_transport_set_tracer()).

Andrew