Adds support for the get rollup job to the High Level REST Client. I had
to do three interesting and unexpected things:
1. I ported the rollup state wiping code into the high level client
tests. I'll move this into the test framework in a followup and remove
the x-pack version.
2. The `timeout` in the rollup config was serialized using the
`toString` representation of `TimeValue` which produces fractional time
values which are more human readable but aren't supported by parsing. So
I switched it to `getStringRep`.
3. Refactor the xcontent round trip testing utilities so we can test
parsing of classes that don't implements `ToXContent`.
Nik Everett