On the test network, we've seen that it becomes harder to connect
if the queues are so short.

With the introduction of static/trusted nodes, the peer count
can go above MaxPeers. Update the capacity check to handle this.
While here, decouple the trusted nodes check from the handshake
by passing a function instead.

Conflicts:
	p2p/server_test.go

This commit changes the discovery protocol to use the new "v4" endpoint
format, which allows for separate UDP and TCP ports and makes it
possible to discover the UDP address after NAT.

This helps with fixing the tests for cmd/geth to run without networking.

The dial timer was not reset properly when the peer count reached
MaxPeers.

removePeer can be called even after listenLoop and dialLoop have returned.

This is supposed to apply some back pressure so Server is not accepting
more connections than it can actually handle. The current limit is 50.
This doesn't really need to be configurable, but we'll see how it
behaves in our test nodes and adjust accordingly.

As of this commit, p2p will disconnect nodes directly after the
encryption handshake if too many peer connections are active.
Errors in the protocol handshake packet are now handled more politely
by sending a disconnect packet before closing the connection.

This commit introduces a new (temporary) peer selection
strategy based on random lookups.

While we're here, also implement the TODOs in dialLoop.

Message encoding functions have been renamed to catch any uses.
The switch to the new encoder can cause subtle incompatibilities.
If there are any users outside of our tree, they will at least be
alerted that there was a change.

NewMsg no longer exists. The replacements for EncodeMsg are called
Send and SendItems.

It is unused and untested right now. We can
bring it back later if required.

They got lost in the transition to rlpxFrameRW.