Compile jdk8

Minor changes so that servo can be built using java 8.

* Disabled javadoc lint checks added in java 8. The comments will need to be fixed if we want to enable them.
* Updated findbugs to 3.0.0 and fix some boxing warnings .
* Fixes bug in CompositeMetricPoller to cancel the future if a timeout occurs. Exposed by minor difference in behavior with java 8.
* Renames spliterator methods of jsr166 classes to avoid compile errors.