Bump PerfMark Min JDK to 8

[carl-mastrangelo]

There have been several things that I'd like to try out with PerfMark, that have have up till now been unavailable due to targetting JDK 6.

• (JDK7) TaskCloseable should really extend AutoCloseable, not Closeable.
• (JDK8) StringFunction supplier should extend Function, or better yet be removed
• (JDK7) Working around Project Loom Thread and Thread local changes would be easier if ConcurrentLinkedDeque was available for ThreadLocal pooling.
• (JDK7) Numerous things that would otherwise be too slow with reflection would be faster if MethodHandles were generally available.
• JDK7 No access to ThreadLocalRandom (Android API level 21). Nice to do occasional cleanup work but not too often. Looks like it can be desugared.
• JDK8 ThreadLocal.withInitial() and java.util.function.Supplier. Nice to have. Would have been nice to add the static method to subclasses of ThreadLocal to hide the parent static method.

I'll add more over time, but these are my wish list.