This change allows returning capitalized HTTP headers, e.g.
`Content-Length` instead of `content-length`.
The HTTP spec says that headers are case-insensitive, but
in fact several clients rely on a specific case in response
headers.
This change will capitalize HTTP headers if the `X-Arango-Version`
request header is sent by the client and contains a value of at
least `20100` (for version 2.1). The default value for the
compatibility can also be set at server start, using the
`--server.default-api-compatibility` option.
* added REST API method HTTP GET `/_api/job/job-id` to query the status of an
async job without potentially fetching it from the list of done jobs
* fixed non-intuitive behaviour in jobs API: previously, querying the status
of an async job via the API HTTP PUT `/_api/job/job-id` removed a currently
executing async job from the list of queryable jobs on the server.
Now, when querying the result of an async job that is still executing,
the job is kept in the list of queryable jobs so its result can be fetched
by a subsequent request.