Java 8 CP2 cleanup

Cleanups

There are some cleanups that could be done after we migrate the JVM requirements from Java 7 to Java 8 CP2.

• https://github.com/eclipse/smarthome/pull/1201 (#3089)
• https://github.com/eclipse/smarthome/pull/2320#issuecomment-253907800
• https://github.com/eclipse/smarthome/pull/2334 (Replace Convertible interface by a default implementation on State)
• Take a look at cron-utils again: https://github.com/jmrozanec/cron-utils (still valid with the new scheduler?)