Scheduler: Refactor nodedb create node

[robertdavidsmith]

Function nodedb.NodeDb.create currently does two things

• Converts schedulerobjects.Node into internaltypes.Node
• Add the node to the db

This PR breaks nodedb.NodeDb.create into two, so instead

• Node conversion is done by internaltypes.Node.FromSchedulerObjectsNode.
• Adding to the nodedb is done by nodedb.NodeDb.AddNodeToDb.

Going forward this will allow us to gradually replace schedulerobjects.Node with internaltypes.Node. Eventually FromSchedulerObjectsNode can be called straight after loading the node from the db, and the scheduler can use only internaltypes.Node internally.