eth/downloader: fix header download limiting

Fixes #2201

Godeps, eth, tests: update ethash, used shared for testing

eth/downloader: bump the download queue size to prevent starvation

Closes #2241: Use Keccak-256 from golang.org/x/crypto/sha3 and mention explicitly

p2p/discover: NTP sanity check clock drift in case of expirations

ethdb: fully disable metrics if not requested (oops?)

Added Vagrantfile so that developers can spin up a VM to build geth

eth: fixed homestead tx check

eth/downloader: fix partial rollback and ancestor lookup

core/state, trie: add node iterator, test state/trie sync consistency

As we aren't really using the standarized SHA-3

This hash function is slightly different from the standarized NIST SHA-3
as both of them use different domain separation bits.

Revision: 1f22c0103821b9390939b6776727195525381532

When a block is queried for retrieval we should add a check whether the
block falls within the frontier rules. If we'd always use `From`
retrieving transaction might fail. This PR temporarily changes
everything to `FromFrontier` (safe!).

core, core/vm: consensus changes necessary for the homestead release

console: seed random number generator

p2p, p2p/discover: EIP-8 changes

Assuming the following scenario where a miner has 15% of all hashing
power and the ability to exert a moderate control over the network to
the point where if the attacker sees a message A, it can't stop A from
propagating, but what it **can** do is send a message B and ensure that
most nodes see B before A. The attacker can then selfish mine and
augment selfish mining strategy by giving his own blocks an advantage.

This change makes the time at which a block is received less relevant
and so the level of control an attacker has over the network no longer
makes a difference.

This change changes the current td algorithm `B_td > C_td` to the new
algorithm `B_td > C_td || B_td == C_td && rnd < 0.5`.

* Removed some strange code that didn't apply state reverting properly
* Refactored code setting from vm & state transition to the executioner
* Updated tests