Feature / Fix: Improve TLS 1.3 support. (#186 by @clue)

TLS 1.3 is now an official standard as of August 2018! :tada: The protocol has major improvements in the areas of security, performance, and privacy. TLS 1.3 is supported by default as of OpenSSL 1.1.1. For example, this version ships with Ubuntu 18.10 (and newer) by default, meaning that recent installations support TLS 1.3 out of the box :shipit:

Fix: Avoid possibility of missing remote address when TLS handshake fails. (#188 by @clue)

Improve performance by prefixing all global functions calls with \ to skip the look up and resolve process and go straight to the global function. (#183 by @WyriHaximus)

Update documentation to use full class names with namespaces. (#187 by @clue)

Improve test suite to avoid some possible race conditions, test against PHP 7.3 on Travis and use dedicated assertInstanceOf() assertions. (#185 by @clue, #178 by @WyriHaximus and #181 by @carusogabriel)