[core] Rework TLS logic

This moves the logic in AbstractXMPPConnection.getSmackTlsContext()
into the ConnectionConfiguration constructor.

Also introduce SslContextFactory and use it in
ConnectionConfiguration.

smack-repl/src/main/java/org/igniterealtime/smack/smackrepl/TlsTest.java

@@ -89,7 +89,7 @@ public class TlsTest {
 
         if (StringUtils.isNotEmpty(tlsPin)) {
             SSLContext sslContext = Java7Pinning.forPin(tlsPin);
-            builder.setCustomSSLContext(sslContext);
+            builder.setSslContextFactory(() -> sslContext);
         }