ServiceContext SPI is not backward compatible

Unfortunately the Java 8 based version of the API uses Optional within the ServiceContext interface, rendering it incompatible with the former version. Since all the rest of the API has this compatibility and also it doesn't hurt to remove Optional here, I heavily propose to change to SPI in the Java 8 API to be the same as in Java 7.
Also this is needed, so many extension modules can similalry work with core implementations in Java 7 AND Java 8.