The OptimizeForXXX methods in FileSystem and Env use ImmutableDBOptions, a non-public struct

There are a set of methods EnvOptions OptimizeForXXX in the Env and FileSystem classes that take in
EnvOptions and ImmutableDBOptions parameters. Unfortunately, the ImmutableDBOptions is not part of the public API for RocksDB (it is defined in the options/db_options.h header).

For consistency, either the ImmutableDBOptions needs to be defined in a public (rocksdb/include) header file, or these methods should instead take a DBOptions object.