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.

eth, eth/downloader: handle a potential unknown parent attack

p2p: tweak connection limits

This should increase the speed a bit because all findnode
results (up to 16) can be verified at the same time.

The returned reason is currently not used except for the log
message. This change makes the log messages a bit more useful.
The handshake code also returns the remote reason.

Introduced default unlock duration when an account is unlocked from t…

p2p/nat: fix UPnP auto discovery