With #714 we added support for cancelling in-flight HTTP requests by putting the response deferred
into an error state. However, this only worked once the underlying TCP connection was
established. With this patch, it is now possible to cancel requests even while the connection is
still being established (possible since Netty 4.1.108.Final via
netty/netty#13849). This also works for `aleph.tcp/client`.

…ment

…tablishment

The new namespace `aleph.util` introduces two new helpers:

* `on-error` which is like `d/on-realized` but only for the error case (success case uses
  `identity`).
* `propagate-error` which propagates error states from a source deferred to a destination deferred
  and optinally accepts a callback which is only run when the propagation has indeed occurred.

These are now used in all places where we propagate error states back to upstream deferreds for the
purpose of cancellation which hopefully makes it a bit more obvious what's going on.

See code comment for background