Add flow protection key caching mechanism for derived keys

The specific algorithm introduced in NIFI-7638 introduces a ~1 sec delay in every encryption operation (which occurs during every flow synchronization and serialization to disk) due to the Argon2 KDF process. This is an acceptable tradeoff for security-conscious users at this time, but can be improved through a key caching mechanism in memory. Deriving the key once at application startup and using it directly will remove this delay, and the key cannot change without an application restart.