module Sequencer:A sequencer is a throttle that is:sig
..end
1. specialized to only allow one job at a time and to not continue on error, and
2. generalized to carry its own state, and enforce mutually exclusive access to that
state by the jobs
type 'a
t
val create : ?continue_on_error:bool -> 'a -> 'a t
val enqueue : 'a t -> ('a -> 'b Deferred.t) -> 'b Deferred.t
val num_jobs_waiting_to_start : 'a t -> int