grpc-java: Bump Guava to version 20

[Eric Anderson]

A year ago grpc-java bumped our dependency on Guava to v20. We then rolled this back as users encountered problems with other libraries being incompatible. This predominantly impacted Cassandra users, but others were possibly impacted. The Cassandra issue was resolved by the Java Driver release 3.2.0, less than a month later.

We'd like to try again to get on Guava 20 and beyond, as newer versions of Guava have stronger API guarantees. Because of precondition optimizations done in Guava 20, grpc-java will require Guava 20+. Unlike our current dependency on Guava 19, it is known-broken to downgrade the Guava version to something older.

If you anticipate this dependency change to cause you problems, please comment on:

https://github.com/grpc/grpc-java/issues/4176