PROTON-697: SChannel functionality: Proton-specific trust stores, client certificates
PROTON-780: Fix "variadic macro" compiler errors under GNU C++ compiler.
PROTON-772: New proton logging code

Based on list discussion:
- moved the log statement api into src/log_private
- simplified the public API to 2 functions: pn_log_enable and pn_log_logger

The minimal public API is required to let applications using proton control
where proton log output goes, which is the point of all this.

The present state of the logging achieves the following goals:
- allow applications to disable or redirect ALL proton library output, nothing is forced to stderr.
- leaves the previous behavior of transport tracers unaffected.
- introduces as little disturbance as possible (one new env. var PN_TRACE_LOG and 2 new API functions)

This is definitely not the final word on logging for proton, but a more
sophisticated, unified logging system requires more discussion in the proton
community.
added link level redirect to pn_condition_is_redirect
PROTON-471: Add Messenger->work to Perl bindings

Added methods for checking that a Messenger instance is blocking. Also
added asynchronous examples for Perl.

Added a recv_async and a send_async example app for Perl, based on the
Python apps.
PROTON-471: Example for Messenger->Work in Perl.
PROTON-471: Reworked how Perl processes error results from API calls.

It does not die on qpid::proton::Errors::INPROGRESS.
PROTON-471: Removed the bool mapping for arguments in the Perl bindings.