Switch to OpenSSL for TLS support on Windows

Proton-C performance has received considerable attention in the last few years resulting in significant performance boosts.  Further improvements are planned and some of these are expected to require non-trivial plumbing changes to the the IO subsystem including TLS support.  Currently a lot of this plumbing has twinned implementations for Windows and Posix.

Given that today the use and adoption of open source software is actively supported by Microsoft, including integrated build tool chains, it makes sense to simplify the Proton code for future enhancements and long term maintenance.

This JIRA tracks ongoing implementation work for the switch from Schannel libraries (native Windows) to OpenSSL.