Fix JavaCase findings

[jtraglia]

PR Description

This touches a lot of files, but the changes are pretty minor. A lot fewer findings than Teku.

• Enable the JavaCase errorprone check.
• Change severity from INFO to WARNING.
• Rename logger (they're all static final) to LOG (simpler than LOGGER).
• Rename all static final variables from camelCase to UPPER_SNAKE.
• Rename local variables from UPPER_SNAKE to camelCase (there were just a few).
• Rename Functions.hkdf_expand to Functions.hkdfExpand.
• Add static qualifier to a few variables that should probably have it.
• Suppress warnings for DiscoveryManagerTest because I think it's easier to read how it is currently.

[ajsutton]

LGTM. Thank you as always. :)