support additional recording_integrity timestamps

These are not actually populated by the code yet. I'm trying to get the
v3 schema frozen as soon as possible; actually using the fields can come
later.

Add some explanation of their value in time.md, along with some general
musing on leap seconds, and a correction on the frequency error of my cameras.

db/upgrade/v1_to_v2.rs

@@ -157,6 +157,8 @@ pub fn run(args: &super::Args, tx: &rusqlite::Transaction) -> Result<(), Error>
         create table recording_integrity (
           composite_id integer primary key references recording (composite_id),
           local_time_delta_90k integer,
+          local_time_since_open_90k integer,
+          wall_time_delta_90k integer,
           sample_file_sha1 blob check (length(sample_file_sha1) <= 20)
         );