Schema: edit obs table

Make subsys a string in case there are ever non-numeric subsystems.
Add a couple more indices.
DB: remove obsreplace option from add_job

It should not be possible for there to be any obs records pointing
to a job which has not yet been added.  Also rename update_obs_table
to set_obs_info.
Remove debugging statement
DB: don't use job_id from obsinfolist

Instead use the value passed as a method argument.  Check that the
obsinfo did not include id or job_id entries.