Newly launched scan not appearing in GET /scans for some time

[mlincett]

A newly launched scan does not appear in the response of GET /scans for some time (minutes to tens of minutes) after the scan is launched. At the same time, the manifest of the scan can be read through the scan_id.

Example:

1. skymist realtime scan-event 138198 44334860 millipede_wilks test 10 => returns scan id ec40f5c59f5c4629a93126e11d788701
2. skymist scan manifest ec40f5c59f5c4629a93126e11d788701 => manifest appears.
3. skymist scan list 138198 44334860 => ec40f5c59f5c4629a93126e11d788701 does not appear in the list until after some time.

[ric-evans]

This was waiting on a field made available after the scan started up. Fixing in #61