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:

• Java compile version issue when upgrading to 3.12.4 protocolbuffers/protobuf#7827
• MOE Sync 2020-08-21 guava#3994
• An alts release built with JDK 9+ wouldn't work under Java 7 or 8 grpc/grpc-java#7348

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

[cpovirk]

Probably at least one affected line:

caliper/caliper-util/src/main/java/com/google/caliper/util/Uuids.java

Line 41 in f1e9f00

buf.flip();

[cpovirk]

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