Improve performance of encryption/decryption mechanism

[swilgosz]

Overview

When we use the encryption transformation for event store, it heavily affects performance by requesting the Encryption key for every single attribute encrypted.

In the systems with large events it can be quite a meaningful overload as ever time we read the stream, all those requests are sent.

Detailed example

I've wrote a blog post about this issue and how I've handled it in the EventStoreCLient gem.

Referenced article: EventStore encryption performance tweaks

Because RailsEventStore is already widely used, adding the completely different encryption schema mechanism can be tricky so I've decided ot add an issue here.

Suggested solution

Add a new encrypted mapper which allows you to define one encryption key for the whole event.

Reason

With one encryption_key per event you can significantly reduce the amount of traffic to the DB when we build projections or read streams.

This will work for small events and for most of the bigger ones.

If there are systems using RES that require multiple encryption keys per event they still can use the existing Encryption Transformation.