Make sure to decrease `num_waiting` when being notified of a peer
failure to allow an additional peer to be queried.

Given that `FixedPeersIter` is initialized with `replication_factor` by
`QueryPool` this bug will not surface today.

* Permit concurrent dialing attempts per peer.

This is a follow-up to libp2p#1440
and relates to libp2p#925.
This change permits multiple dialing attempts per peer.
Note though that `libp2p-swarm` does not yet make use of this ability,
retaining the current behaviour. The essence of the changes are that the
`Peer` API now provides `Peer::dial()`, i.e. regardless of the state in
which the peer is. A dialing attempt is always made up of one or more
addresses tried sequentially, as before, but now there can be multiple
dialing attempts per peer. A configurable per-peer limit for outgoing
connections and thus concurrent dialing attempts is also included.

* Introduce `DialError` in `libp2p-swarm`.

For a cleaner API and to treat the case of no addresses
for a peer as an error, such that a `NetworkBehaviourAction::DialPeer`
request is always matched up with either `inject_connection_established`
or `inject_dial_error`.

* Fix rustdoc link.

* Add `DialPeerCondition::Always`.

* Adapt to master.

* Update changelog.

Instead of creating unconstrained random number generators in quickcheck
tests to generate test data, have quickcheck provide a `Seed` to seed
those random number generators and thus make the test execution
deterministic / reproducible.

…ibp2p#1573)

`FixedPeersIter` requires the initial set of peers to be passed as
`PeerId`s and not as `Key<PeerId>`s. This commit removes the unnecessary
conversion.

… bytes (libp2p#1570)

* Update to latest multihash.

* Update changelog.

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>

…#1574)

* Make the number of events buffered to/from tasks configurable

* Assign a PR number

* Fix comment

* Apply suggestions from code review

Co-authored-by: Roman Borschel <romanb@users.noreply.github.com>

* Rename variables

* Apply suggestions from code review

Co-authored-by: Roman Borschel <romanb@users.noreply.github.com>

Co-authored-by: Roman Borschel <romanb@users.noreply.github.com>