Implement events log

[smt116]

Add the mechanism for collecting and reading the following events:

• "John Doe registered" (user created)
• "John Doe updated the account" (user updated)
• "John Doe deleted his account" (user deleted)
• "John Doe added a new post" (post created)

These are high-level descriptions that could be displayed by the client application. From the internal system, it needs to be possible to see more details about a given event (for example, which fields of the user were updated or what e-mail address has been used to add a new account).

All events need to be persistent. Also, they have to be stored in the separate data store from the main database.

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Summary:

• Use a separate data store for events (it can be any engine - there is no need to worry about the performance)
• Events need to be persistent
• Collect events listed at the beginning
• Expose API for reading events