Improve support for cancellation

[mzimbres]

At the moment the connection classes provide support to cancellation with cancel member function. Although not common or even advisable, some users may want to cancel operations with timeouts like this

co_await (conn.async_run(...) || timer.async_wait(...));

This is working as long as the timeout is not too small e.g. 10ms. To handle that properly Aedis need to improve cancelation support. Below some parts of a conversation I've had with Richard

timer waits and IO reads will be cancellation-aware. But an operation that you have written yourself may not be.

these operations may want to check the cancellation context hasn't already been cancelled before initiating, and also store a cancellation handler in connected cancellation slots in order to cut short any long-standing operation. asio coroutines do this pre-initiation check for you https://github.com/boostorg/asio/blob/0af7858e7b5603a2415a30b69e16c7ef1d47a5e9/include/boost/asio/detail/deadline_timer_service.hpp#L251

also, If you cancel after posting an operation, when the posted code is executed, it may want to check whether the cancellation_state has been cancelled while the posted step was waiting in the executor.

// some code...
asio::post(exec, some_next_step(std::move(my_handler));
// cancellation happens here...
// some_next_step will not know that the handler was cancelled unless it checks.
// ...
// ... some_next_step now executes...

void some_next_step(auto handler)
{
  auto cs = get_canecllation_state(handler);
  if (cs.cancelled() != asio::cancellation_type::none)
  {
    complete(handler, operation_aborted);
    return;
  }
  // ... not cancelled
}

[mzimbres]

Motivation for this issue was #28.

[mzimbres]

All forms of cancellation are now supported, for example, implicit with operator || and && or explicit with the cancel member function.