If a peer fails to respond (disconnect, etc) during hash downloading
switch to a different peer which has it's current_hash in the queue's
peer set.

The downloader is no longer responsible for processing blocks. The
eth-protocol handler now takes care of this instead.

Added graceful shutdown during block processing. Closes #846

Peers in the eth protocol handler are now being ignored for catch up.

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