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.
Can we note, perhaps parenthetically, which garbage collector is the default? In general, assume that users know nothing about java, so give them guidance based on default settings before noting advanced considerations.