Switch to ValueTask<T> and refactor Notify > NotifyAsync

[kzu]

There are a number of minor drawbacks to the current IMessageBus interface:

1. ExecuteAsync forces an allocation even when commands complete synchronously (see https://devblogs.microsoft.com/dotnet/understanding-the-whys-whats-and-whens-of-valuetask/)
2. Notify cannot involve async behavior (i.e. persisting events to a queue or cloud)

For that reason, we'd like to make the following changes:

ValueTask ExecuteAsync(...)
ValueTask<TResult> ExecuteAsync<TResult>(...)
ValueTask NotifyAsync<TEvent>(...)

Except for the last, the other two should be mostly source-compatible if the consumers are simply awaiting the result.

For legacy code using Notify synchronously, an extension method could run NotifyAsync and spinwait on IsCompleted, say.