Better support for request cancellation: add context.RequestAborted

[exyi]

• New API makes it easier (and faster) to obtain the CancellationToken in user code. Since the token should generally be propagated to all async methods, it can simplify a lot of code.
• The property name is the same as in Asp.Net Core
• DotvvmPresenter will check the RequestAborted token in strategic places and stop executing the request:
  • before each lifecycle event is called on the control tree
  • before command execution
  • in error handling: user exception handling is disabled, except for Dispose methods

[tomasherceg]

We should remove the IRequestCancellationTokenProvider interface and its implementations.