With this commit we change the data type of the 'TIMESTAMP'
meta-data field from a formatted date string to a plain
`java.util.Date` instance. The main reason for this change is
that our benchmarks have indicated that this contributes
significantly to the time spent in the ingest pipeline.
The overhead in terms of indexing throughput of the ingest
pipeline is about 15% and breaks down roughly as follows:
* 5% overhead caused by the conversion from `XContent` -> `Map`
* 5% overhead caused by the timestamp formatting
* 5% overhead caused by the conversion `Map` -> `XContent`
Relates #22074
Daniel Mitterdorfer