MINOR: sample: add the "master_id" and "worker_id" sample fetches

MINOR: sample: add the "master_id" and "worker_id" sample fetches

These two new sample fetch functions return the unique identifiers of the
master and of the current worker process, so that they can be placed in
logs, in response headers, or be used in any other expression.

The "master_id" one fails when there is no master, i.e. when not running
in master-worker mode, so that a log or a header will simply not report
anything there instead of an inaccurate value.

Note that these functions rely on internal storage, not environment
variables, so they are unaffected by "setenv" directives.