instantaneous sync can cause outbox to have incorrect state

[sheppard]

So far this has only surfaced in the test environment (where network traffic is effectively instant). The process is something like this:

1. Store two or more unrelated records in the outbox while offline (or with backgroundSync set to -1)
2. Trigger the sync. Since the records are unrelated (i.e. no foreign keys to each other), they are sent in parallel.
3. When the sync completes, both calls to sendItem() retrieve and update the outbox state for their respective item.
4. If the records are successfully synced within milliseconds of each other, the last write to the outbox will overwrite the previous one.
5. Thus, only one of the records is marked as synced, when actually both were successful.

This can be "fixed" in the test environment by adding a small delay to the test server (45cd0c3).

Some better options include;

• Lock the outbox state while writing and/or minimize the time between read and write
• Store sync statuses in memory or in separate storage keys
• Serialize sync process so only one record is sent at a time (c.f. send parents first and update foreign keys with sendAll #95)

[sheppard]

This will be fixed by using redux-offline (#105).