Client: Implement Transactional Outbox

[Zhiyuan-Amos]

After writing to Outbox, either:

1. Js calls a .NET method to Notify State Changed
2. .NET calls Notify State Changed.
   To trigger code to process Outbox.

Also, consider periodic processing of Outbox in case some of the processing fails. However, this will introduce race conditions, so for simplicity, we could consider processing Outbox on app launch.

(Evaluate: Use MessageId, AggregateId, AggregateType, EventType, Payload)

Writing to Server should occur before writing to Client. See #93