This commit changes the date handling. First and foremost Elasticsearch
does not try to convert every date to a unix timestamp first and then
uses the configured date. This now allows for dates like `2015121212` to
be parsed correctly.
Instead it is now explicit by adding a `epoch_second` and `epoch_millis`
date format. This also means, that the default date format now is
`epoch_millis||dateOptionalTime` to remain backwards compatible.
Closes#5328
Relates #10971
01e8eaf181