Add optimistic concurrency to samples

[oskardudycz]

Added optimistic concurrency to samples and did a huge all-around refactoring.

The most significant changes:

• Added OptimisticConcurrencyMiddleware and related classes to support full flow based on the ETag. Applied both for Marten and EventStoreDB samples,
• Aligned convention around Records (use them for DTOs with static factory method for validation),
• Introduced StreamEvent for EventStoreDB subscriptions to gather information about stream revision and global position. That can be used for idempotency checks in projections.
• Made MartenExternalProjection, EntityFrameworkProjection idempotent,
• Merged EventStoreDB improvements from Simple EventStoreDB examples into Core project,
• Added NoMediatorEventBus to not rely on the marker interfaces need etc.
• Aligned ECommerce samples:
  • API structure and models
  • The same set of integration tests,
  • Naming and structure conventions
• Added CorrelationIdMiddleware and plugged it initially. More changes will come in the follow-up PR,
• Unified background processing for ESDB Subscriptions and Kafka Consumers

Fixes #50

[mehdihadeli]

Hi @oskardudycz ,
Hope you are doing well :)
I saw your pull request about concurrency with using ETag, it was very interesting approach.
I have a question, What about using aggregate version property for handling concurrency? As you have it in your code, but you didn't use it for handling concurrency. Using this version is not simpler?