ENH: Allow RunEngine customize scan_id calculation
ENH: Allow RunEngine customize scan_id calculation

Before the RunEngine will increase 1 each time for
scan_id. There isn't each way to change this scan_id
calculation method.
Since scan_id was created as a user-friendly number,
Now there is a callable argument scan_id_source to
allow user pass in a bespoken calculation method for
scan_id.

ericdill/bluesky,ericdill/bluesky