Closes #5

Split the type variable of the protocol into two for different return
types of the `get_update` and `completed` methods.

Rename `konnect.curl.abc.Request` methods:

- `has_response()` -> `has_update()`
- `response()` -> `get_update()`

Closes #1

- Move methods that didn't need to be methods out of Mutli class.
- Use individual send/recv streams instead of a stapled stream, as
  these type check better; send end needs contravariant argument type,
  recv end needs covariant return type, stapled stream could only be
  invariant.

Intermediate responses were being ignored until at least one request
was fully complete.

Mostly for the typing improvement made to `create_memory_object_stream`

Rather than a millisecond delay to sleep for at some point in the
future, '_deadline' is an absolute time according to the monotonic
process clock to sleep until, in millisecond precision.

Give the speed at which the timer is likely to be started after setting
the time it will not make a lot of difference to the timing accuracy.

Once (if) the following is merged and released, this can be reverted
and the relevant release made a dependency:
  https://github.com/pycurl/pycurl/pull/803