SslBump: Disable OpenSSL TLSv1.3 support for older TLS traffic (#588)

SslBump: Disable OpenSSL TLSv1.3 support for older TLS traffic (#588)

This change fixes stalled peeked-at during step2 connections from IE11
and FireFox v56 running on Windows 10 (at least), producing "Handshake
with SSL server failed" cache.log errors with this OpenSSL detail:

`SSL routines:ssl_choose_client_version:inappropriate fallback`

Disabling TLS v1.3 support for older TLS connections is required
because, in the affected environments, OpenSSL detects and, for some
unknown reason, blocks a "downgrade" when a server claims support for
TLS v1.3 but then accepts a TLS v1.2 connection from an older client.

This is a Measurement Factory project