Lookup calls would spin out of control when network connectivity was
lost. The throttling that was in place only took effect when the table
returned zero results, which doesn't happen very often.
The new throttling should not have a negative impact when the host is
online. Lookups against the network take some time and dials for all
results must complete or hit the cache before a new one is started. This
usually takes longer than four seconds, leaving online lookups
unaffected.
Fixes#1296
The most visible change is event-based dialing, which should be an
improvement over the timer-based system that we have at the moment.
The dialer gets a chance to compute new tasks whenever peers change or
dials complete. This is better than checking peers on a timer because
dials happen faster. The dialer can now make more precise decisions
about whom to dial based on the peer set and we can test those
decisions without actually opening any sockets.
Peer management is easier to test because the tests can inject
connections at checkpoints (after enc handshake, after protocol
handshake).
Most of the handshake stuff is now part of the RLPx code. It could be
exported or move to its own package because it is no longer entangled
with Server logic.
