Introducting synchronous triggers.

[koenbeuk]

This PR adds support for synchronous triggers. As a result, existing triggers types are renamed to properly reflect their async nature. As an example, we now expose:

// Newly added
interface IBeforeSaveTrigger<TEntity> {
  void BeforeSave(ITriggerContext<TEntity> context);
}

// Renamed, used to be IBeforeSaveTrigger<TEntity>
interface IBeforeSaveAsyncTrigger<TEntity> { 
  Task BeforeSaveAsync(<TEntity> context, CancellationToken cancellationToken);
}

This PR will also introduce a proper synchronous path without async-over-sync. Calling dbContext.SaveChanges() will only call synchonous triggers. Both synchronous as well as asynchronous triggers are called when calling dbContext.SaveChangesAsync(). We should consider warning the user when SaveChanges() are being called while asynchronous triggers exists.

As a consequence of this change, the TriggeredDbContext base class has been removed.

This PR will break things!