A table of sequencers indexed by key, so that at any moment for each key there is at most one job running.

An 'a Sequencer_table.Make(Key).t is similar in concept to:

      { mutable state : 'a option;
        jobs  : 'a option Throttle.Sequencer.t;
      } Key.Table.t

It allows one to run jobs that are indexed by a key, while allowing jobs with distinct keys to run simultaneously, and ensuring that for any given key, at most one job with that key is running at a time. A sequencer table maintains optional state for each key, and supplies that state to each running job indexed by that key.

The implementation of a sequencer table is optimized for having a large number of keys with state, while only a few keys with active jobs at any given time. So, it is implemented with two tables, one of states and one of sequencers:

      { states : 'a Key.Table.t;
        jobs   : ('a option -> unit Deferred.t) Sequencer.t Key.Table.t;
      }

The implementation automatically adds a sequencer to the jobs table, if necessary, when one adds a job, and automatically removes a sequencer from the jobs table whenever the sequencer has no jobs to run.

The implementation does not automatically release state; one must call set_state t ~key None.

module Make : 
functor (Key : Core.Std.Hashable) -> sig .. end
type 'a t
val create : unit -> 'a t
val enqueue : 'a t ->
key:Key.t -> ('a option -> 'b Import.Deferred.t) -> 'b Import.Deferred.t
enqueue t ~key f enqueues f for key. f will be called with the state of key when invoked.

Invariant 1: it is guaranteed that f will not be called immediately.

Invariant 2: if f raises, then the exception will be raised to the monitor in effect when enqueue was called. Subsequent jobs for key will proceed.

Invariant 3: to avoid race, there are no deferred operations between finding the state and calling f with the state found. Otherwise, the user would need to consider the race that the state passed to f might have been changed by set_state.

val set_state : 'a t -> key:Key.t -> 'a option -> unit
set_state t key state_opt sets the state for key immediately. The state will be kept internally until set to None
val find_state : 'a t -> Key.t -> 'a option
val num_unfinished_jobs : 'a t -> Key.t -> int
num_unfinished_jobs t key returns the number of jobs for key including including pending and running.
val mem : 'a t -> Key.t -> bool
mem t key returns true if there is state or an pending/running job
val fold : 'a t -> init:'b -> f:('b -> key:Key.t -> 'a option -> 'b) -> 'b
Fold over keys with states or pending/running jobs. It's safe to mutate (enqueue or set_state) when folding