Introduce cache.memory_ratio configuration for HPCache

New set-and-forget configuration that specifies cache size as a ratio of the
old generation max size. Memory division across internal components of the cache
is handled by fixed ratios.

If any of the old config (now referred to as "advanced config") is set, it will
override the memory_ratio configuration.
I like the Java 8 `java.util.Optional`, especially how it is very inlining friendly and a super simple implementation by being a final class.