Add modes for migrating data to SS

[kurtisnelson]

When migrating to SS, you often have a step where writes go to both SS and the legacy storage. In this state, the SS is rarely used for performant read operations and you likely have some sort of memory cache in the legacy storage. If the data is especially big, writes greatly outnumber reads, or the legacy storage framework is causing OOMs, a mode turning off in-memory write caching is useful.

I propose two new configurations are added:
NamespaceConfig.WRITE_HEAVY
NamespaceConfig.WRITE_HEAVY_CACHE

In this mode, a write invalidates the LruCache entry instead of updating it. Reads would still populate the (likely small) cache.