Support JDK 15 and Guava version 29.0-jre

JDK 15 is now GA (released 2020/09/15). We should support it (and OpenJDK 15) as a build- and run-time platform.

Also:

• Upgrade to latest Guava (guava-29.0-jre, released on 2020/04/13);
• Upgrade log4j2 to latest (2.13.3, release 2020/05/10);
• Remove log4j (we're on the last release, 1.2.17, which was 2012/05/26, but I assume the project is now unsupported and dead).

If there are any other important upgrades to be done before 1.26, please add them here.

Must fix in next release (1.26) or justify why not in this thread.