Before this change, the transition to the reset state wouldn't notify
tasks that were waiting for a response. The motivating case for this
patch involved a large header being sent by the server. This case was
mostly tested by an existing test, but because that test did not spawn
separate tasks and kept polling the futures through its use of
`conn.drive`, the missing notify was masked.

Informs hyperium/hyper#3724.

…ers (#792)

The push task is a separate task from the recv task, so its state needs
to be tracked separately for waking. I don't know how to be systematic
about ensuring that notify_push is called in all the right places, but
this is an initial attempt.

In order to test this works, we manually utilize FuturesUnordered which
does fine-grained task wake tracking. The added test failed before
making the other changes.

This commit adds wrappers around futures::future helpers and augments
TestFuture to ensure that the underlying futures are notified before
they are polled. This helps to catch bugs where there are missing notify
calls or bad handling of the waker.

The commit then extends the tests to use these helpers instead of the
library functions from futures.

It also ammends the client_requests::recv_too_big_headers test to no
longer use the tokio spawned tasks that were added in #791.