12167: Improve request-id tracking in Workbench.

12167: Improve request-id tracking in Workbench.

Include current request ID in API requests via X-Request-Id header
instead of a form parameter.  (Also move Arvados API token from form
to Authorization request header.)

Always log request ID as its own field, instead of adding it to params
(where it might get truncated before being logged).

Use new request ID format: "req-" + 20-random-alphanum-chars.

Use an around_filter to set/clear the current request ID in one place,
instead of setting it in controller code and clearing it in logging
code.

Return request ID to browser via X-Request-Id header.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tclegg@veritasgenetics.com>