avoid time stamp confversion at every request

The reason string with time stamp does not need to be parsed more often
than at the times of changes.  This should make serving requests quicker.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
clean up header file

Commit e66c82a5 added few unnecessary enum members to f5gs.h header.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
clarify state code related processing

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
include delta time to why printing

Who ever is interested of when status changed the same person is likely
interested how long the state has been like that, so give the information
as default even if it is sometimes unnecessary.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>