Api: Disallow UD of existing entries if partner has performed UD on that entry

[Zhiyuan-Amos]

Problem: When A updates a particular entry, and B also updates that entry prior to running synchronization, the data might go out of sync. This happens when updating Aggregate Roots.

The solution is to create a Container for each Domain, and the Container stores the data temporarily. Note that if-statement to verify if the data has been modified is subject to race conditions, so the implementation should be to set the EntityId as the unique id for the Container.

This requires de-coupling Domain & Change: each DomainFunction stores the content temporarily, which is cached in Change. On Change deletion, ChangeFunction sends Event so each respective Service deletes the value.

Note:

1. Self can update the same content multiple times.
2. Done Id (i.e. DateTime) has to be checked for concurrency too.
3. Server could return a custom HTTP Response Code, such that Client would perform a synchronization first before redo-ing the operation.

Related: Issues should have unique titles.