[core/swarm] Refactor and extend configurable connection limits.

[romanb]

The initial motivation for the work in this PR is #1839. However, the PR goes a bit further than to just add a single counter to have a more streamlined API. Thus, in order to better track different connection counts and permit configurable limits for these counts and make these available for inspection in an efficient manner (i.e. without linear counting operations), I introduced dedicated connection counters via a ConnectionCounters structure maintained by the internal connection pool but exposed on the API via the NetworkInfo. All connection states that are counted in this way can also have effective configurable limits.

All limits are now configured through a ConnectionLimits structure. The new configurable limits are

• ConnectionLimits::max_established_incoming for limiting the number of concurrent established connections that were incoming.
• ConnectionLimits::max_established_outgoing for limiting the number of concurrent established connections that were outgoing.

A limit that has been removed because tracking the related counts efficiently would be a bit laborious and I don't think it is particularly useful is

• NetworkConfig::set_outgoing_per_peer_limit()

Note that there are still the pre-existing ConnectionLimits::max_incoming and ConnectionLimits::max_outgoing which correspond to the two new configuration options above, just for pending connections. I also took the liberty to change the connection limits to use u32 instead of usize for their types, which seems more appropriate.

Fixed panic in the Peer API

The Network::peer() API allows to iterate mutably over the ongoing connection attempts via DialingPeer::attempts(), where each DialingAttempt can be abort()ed during iteration. However, the iterator did not take into account the changes to the internal iteration offsets when elements are removed. This was discovered while extending the connection limit tests a bit and has been fixed now as a result.

[mxinden]

Good catch on the iterator state mismatch!

[tomaka]

Looks good to me with the same remarks as Max on the naming.

[romanb]

I incorporated the naming suggestion(s) in 4e447e9 together with a bit of refinement for the configuration APIs:

• ConnectionLimits now has private fields for future extensions and offers a builder API that moves self.
• NetworkConfig now has a builder API that moves self.

The SwarmBuilder already used this style, so there is some gain in consistency. Let me know if that last commit looks acceptable.

[mxinden]

Both the connection limiting as well as the general API improvements look good to me.