Add method toInternalByteRef to FieldType and possibly deprecate toInternal()

Point FieldTypes can't be Strings in it's "internal representation", so calling ft.toInternal(...) is not supported. For those fields we use toInternalByteRef(...) that does the same thing but returns a BytesRef instance instead of a String. In many cases, the consumer of toInternal(...) actually needs (or can optionally use) a BytesRef and we end up creating one from the String. It would be better to have toInternal() return a BytesRef instance, it can be easily converted to a String if needed