Store epoch timestamp when writing Identity Entities

# Overview

We need to be able to tell whether an Identity has changed, and if the entity that is being stored in memory is stale and needs refreshing. The two current use cases for this requirement are:

- With real time updates, where a client loses then regains a network connection, ascertaining if they have missed an update event message
- With Edge API replication, race conditions can occur where replication lags and clients attempt to update their documents too quickly

# Solution

Whenever an Identity is modified/updated/written to, store the current per-second UNIX epoch time against the Identity.

When sending an Identity to an SDK, send the last-updated epoch time to the SDK. The SDKs can then make use of this data to decide whether they need to refresh their data.

Also related is https://github.com/Flagsmith/flagsmith/issues/1420