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

Miner updates and downloader events

Omit replies for RPC notification requests

When Id is missing, the client does not want a response

core: changed log message for forks. closes #952

Format contract code as hexdata

Check for error on DB fetch of tx

cmd/geth: use plain keystore for JS tests

This reduces the time it takes to run the tests back to 3 seconds.