Function catch_up 

pub fn catch_up()

Let the jobs queued before this call apply first, for a reader that acts on the book or the claims: a finish, a start, a commit’s book. Waits at most [CATCH_UP]; a job still queued then applies later. Jobs queued after the call began are not waited for, so a burst cannot hold the reader. Inside a hook or a drainer this returns at once, since the job it would wait for may be its own.