sync: add `Notify::notify_waiters` (#3098)

This PR makes `Notify::notify_waiters` public. The method
already exists, but it changes the way `notify_waiters`,
is used. Previously in order for the consumer to
register interest, in a notification triggered by
`notify_waiters`, the `Notified` future had to be
polled. This introduced friction when using the api
as the future had to be pinned before polled.

This change introduces a counter that tracks how many
times `notified_waiters` has been called. Upon creation of
the future the number of times is loaded. When first
polled the future compares this number with the count
state of the `Notify` type. This avoids the need for
registering the waiter upfront.

Fixes: #3066

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