Update metadata translation to know about observing_day and sequence number

• By default observing_day will by an integer YYYYMMDD derived from the UT date of observation (this is a good default for Hawaii)
• obs_lsst will override it rom the DAYOBS definition
• sequence_number will be SEQNUM for obs_lsst. I'm not sure what the default should be for other instruments. I can think of two options: define it to be sequence in observing_day but 0 if unknown; or else allow it to be the "odometer" counter if that is more convenient (which is the same as the exposure_id for HSC and CFHT.

In a follow up ticket these will be added to butler registry.