altoslib: Use sequence numbers to track GPS updates to AltosRecord

State objects now record what GPS sequence ID they have to know when
the GPS data has been updated. Record objects bump the GPS sequence
each time new GPS data is recorded. This way, record objects aren't
modified as they're iterated over to generate the list of state
objects which makes it possible to iterate multiple times and get the
same resulting set of states.

Signed-off-by: Keith Packard <keithp@keithp.com>