* Refactors ClientHelper to combine header logic
This change removes all the `*ClientHelper` classes which were
repeating logic between plugins and instead adds
`ClientHelper.executeWithHeaders()` and
`ClientHelper.executeWithHeadersAsync()` methods to centralise the
logic for executing requests with stored security headers.
* Removes Watcher headers constant
When validating the search request, we make sure any date_histogram
aggregations have timezones that match the jobs. But we didn't
do any such validation on range queries.
While it wouldn't produce incorrect results, it would be confusing
to the user as no documents would match the aggregation (because we
add a filter clause on the timezone for the agg).
Now the user gets an exception up front, and some helpful text about
why the range query didnt match, and which timezones are acceptable
Colin Goodheart-Smithe
Zachary Tong
Ryan Ernst