The previous metric was pubkey1^pubkey2, as specified in the Kademlia
paper. We missed that EC public keys are not uniformly distributed.
Using the hash of the public keys addresses that. It also makes it
a bit harder to generate node IDs that are close to a particular node.

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.

eth/downloader: missing parent improvement

p2p.Msg.ReceivedAt can be used for determining block propagation from
begining to end.

When blocks have been sorted and are being processed, orphan blocks
should be ignored and thrown out. The protocol handler is responsible
for downloading blocks which have missing parents.

core: chain fork fix

Makefile improvements

Chain reorgs weren't properly handled when a chain was further ahead.
Previously we'd end up with mixed chains in our canonical numbering
sequence. Added test for this type of forking.

```
     /-o-o-o   A
o-C-+
     \-o-o-o-o B
```

Ends up with with C A1, A2, A3, B4

Insert blocks will no longer allow processing of multiple chains at the
same time. The block lock has been moved to start of the function.

Remove path separator literals

The default value of the Shh option is true, but cli can't
see it because it is not part of the mist options.

This is necessary because cli doesn't see the default
value unless the flag is included.

This ensures that compiler error messages contain the correct path.

p2p/discover: fix api issues caused by leveldb update

rlp: fix list bounds check overflow (found by go-fuzz)