The existing approach used date formatters when a format based string
like `date_time||epoch_millis` was used, instead of the custom code.

In order to properly solve this, a new interface called
`DateFormatter` has been added, which now can be implemented for custom
formatters. Currently there are two implementations, one using java time
and one doing the epoch_millis formatter, which simply parses a number
and then converts it to a date in UTC timezone.

The DateFormatter interface now also has a method to retrieve the name
of the formatter, which is needed for mapping changes anyway.

The existing `CompoundDateTimeFormatter` class has been removed, the
name was not really nice anyway.

One more minor change is the fact, that the new java time using
FormatDateFormatter does not try to parse the date with its printer
implementation first (which might be a strict one and fail), but a
printer can now be specified in addition. This saves one potential
failure/exception when parsing less strict dates.

If only a printer is specified, the printer will also be used as a
parser.