Improve EntityStorage behavior on non-primitive types

[MHHukiewitz]

• To extend on the BN issue from Provide standard middlewares for Blockchain primitive types #19, it might be useful to provide default mapFn implementations for parsing BN values from LevelDB (which are stored there as hex strings). This implementation should also scan for nested fields of the stored entities and parse them, if required.
• The filterFn turned out to be unnecessary, as it is nowhere used, and may be removed.
• An additional function may be defined to define how certain fields should be serialized into the DB, which may be used in conjunction with mapFn to obtain full control over SerDe when communicating with LevelDB.