Check for problems with ByteBuffer covariant return types when building with JDK 9+

[cpovirk]

Caliper uses ByteBuffer, so building it with JDK 9+ may make it fail at runtime under Java 8 or lower, similar to:

• https://github.com/protocolbuffers/protobuf/issues/7827
• https://github.com/google/guava/pull/3994
• https://github.com/grpc/grpc-java/issues/7348

But I haven't checked any more detail than that.

[cpovirk]

Probably at least one affected line:

https://github.com/google/caliper/blob/f1e9f00db3233914056d168e9bded0d84fb73e59/caliper-util/src/main/java/com/google/caliper/util/Uuids.java#L41

[cpovirk]

(#458 will fix the current error, but I'll leave this open to track automatically detecting any future such problems.)