Cooldowns: add missing docs, implement min-zero time difference, improve tests

[CosimoTiger]

• add missing class documentation
• implement Maximum zero time return alternative for ICooldown.getRemaining() #24 by renaming getRemaining into getDifference and adding getRemaining which returns time until expiration, minimally zero
• rename Cooldowns.cleanup to Cooldowns.clearExpired for better clarification
• add getDifference to the tests
• improve some parts of the tests
• make the cooldown tests concurrent again because some of them are using Thread.sleep