custom formatters for partitioning

[d80tb7]

WIP

Things I don't like:

1. We're still calling convertSpecialDate/Timestamp just as much as the previous solution as it's called twice in the parse() methods.
2. DateFormatter.getPartitioningFormatter and TimestampFormatter.getPartitioningFormatter are duplicating the legacy format stuff in DateFormatter.apply and `TimestampFormatter.apply``` respectively.

First could be solved by extracting the parse logic from e.g. Iso8601DateFormatter into some mthod with a considerSpecial flag. This would come at the cost of the code more difficult to read. E.g. we might need an extra abstract base class.

Second can only be solved if we decide we don't need to support legacy formats, but this would be a breaking change with the previous code.