This commit changes the default version of Tomcat to 8.5.3 while
also retaining support for Tomcat 8.0 and 7.0. The main difference
in 8.5 is that the ServerSocketFactory abstraction that allowed the
TrustStore and KeyStore to be configured programatically no longer
exists. This logic has been replaced with the use of a custom URL
protocol (springbootssl) that provides access to the key store and
trust store of an SslStoreProvider. In addition to working with 8.5,
this approach has the advantage of also working with 8.0 and 7.0.
Closes gh-6164
Andy Wilkinson