Revert #1409 partially (#1603)

Futures channels that are used by default has a side effect of
`Sender::Clone` that efficiently increases the capacity of the bounded
channel by one. This PR fixes the undesired backpressure removal that
was caused by the #1409. This issue has been discovered by @sandreim
during Versi testing and needs to be treated as critical that should not
be included in any release without this reversion.

This PR reverts the original behaviour.