ChainManager now uses a parallel approach to block processing where all
nonces are checked seperatly from the block processing process. This
speeds up the process by about 3 times on my i7

core: global chain insert lock

cmd/geth: delete state db on upgradedb command

Start event has moved because it could possibly could stall the miner

core, miner: fork resolving and restart miner after sync op

Fork resolving fixes #940

eth/downloader: circumvent download race between crosscheck and hashes

eth/downloader: fix #992, where tests may time out on a slow machine

Update state tests and vm create return

eth/downloader: fix cancel channel double close

Fix downloader vulnerabilities