Implement outbox system

[chrisbobbe]

Something like this, maybe. From discussion:

            User cancels the scheduled send.
           ┌────────────────────────────────────────────────────────────────────────┐
           │                                                                        │
           │                                                                        │
           │                                   User cancels during send (#4170).    │
           │                                  ┌───────────────────────────────────┐ │
           │                                  │                                   │ │
           │                                  │                                   │ │
           │                                  │                Event received,    │ │
(create)   │      Time for the scheduled      │                or we abandoned    │ │
  │        │      send (a try/retry).         │    200.        the queue.         ▼ ▼
  └► should-send ───────────────────────► sending ─────► sent ────────────────► (delete)
       │ ▲ ▲                                │ │                                     ▲
       │ │ │                                │ │                                     │
       │ │ │ App quit: schedule auto-retry. │ │                                     │
       │ │ │                                │ │                                     │
       │ │ │ 5xx, network error, or (with   │ │                                     │
       │ │ │ #4170) 60s network timeout:    │ │                                     │
       │ │ │ schedule auto-retry with       │ │                                     │
       │ │ │ backoff.                       │ │ 4xx.                                │
       │ │ └────────────────────────────────┘ └───────────────────────┐             │
       │ │                                                            │             │
       │ │                                                            │    User     │
       │ │ User requested a retry; schedule to run immediately.       ▼    cancels. │
       │ └──────────────────────────────────────────────────────── failed ──────────┘
       │                                                              ▲
       │                                                              │
       │ Too old: message is "better never than late". Too much time  │
       │ from creation or last user-requested retry.                  │
       └──────────────────────────────────────────────────────────────┘

[gnprice]

From a user perspective this feature means:

• We retry sending the message, if we can't reach the server at the moment you first try (perhaps you're on a train in a tunnel, or on the road with spotty cell connections) but are able to sometime soon after, like minutes after.
• When we give up on sending the message, we give the user feedback so they know it wasn't sent.
• When we've given up on a message, we give the user the option to retry sending it.

[gnprice]

An implementation note: this is what the method PerAccountStore.sendMessage is for.

Currently that method does nothing other than make the underlying sendMessage API request. When we implement this issue, the same method will instead create a new outbox-message in the subsystem described above.

As a matter of code organization, that outbox data will probably want to live on its own class defined in its own file; PerAccountStore will just hold onto an instance of that class, and PerAccountStore.sendMessage will delegate to that. See Inbox and StreamStore for existing examples (showing two somewhat different versions of that pattern).

[gnprice]

An initial, simplified version of an outbox system is now planned for before launch:

• Simplified local echo, with retry #1441