Ensure serial number is up-to-date when shipping logs

[RebeccaMahany]

The logshipper pulls the serial from ServerProvidedDataStore, but that store doesn't get updated when launcher is transferred to a new computer -- so we continue to ship logs with the old serial attached to them. This makes it hard to troubleshoot issues when launcher is transferred to a new computer. (Related: #1346.)

We should make sure the serial is always up-to-date when shipping logs. I see a couple options:

• Use knapsack.GetEnrollmentDetails() => HardwareSerial. This has the drawback of not being set yet when launcher first starts up, so we'd need to handle that scenario:
  • We could delay shipping logs until the serial is available
  • We could use ServerProvidedDataStore's serial initially and then update to use knapsack.GetEnrollmentDetails() => HardwareSerial once available
  • We could start persisting EnrollmentDetails in a db store so that they'll be available right away
• Query for the serial directly via runsimple

Audit anywhere else we use ServerProvidedDataStore().Get([]byte("serial_number")) and see if we should update to use the enrollment details instead.