Improve export command 

core: added a test for missing nonces

This test showed the logic in the queue was slightly flawed sending out
transactions to its peer it couldn't even resolve itself.

core: settable genesis nonce

crash fix: skip deep log if self.chain is not caught up

@see trace https://gist.github.com/eupraxic/87fdfefe702c51d5944d

You can set the nonce of the block with `--genesisnonce`. When the
genesis nonce changes and it doesn't match with the first block in your
database it will fail. A new `datadir` must be given if the nonce of the
genesis block changes.

core: fix nonce verification one more time

The block nonce verification was effectively disabled by a typo.
This time, there is an actual test for it.

eth/downloader: handle timeouts more gracefully

eth, eth/downloader: fix #1098, elevate empty hash errors to peer drops

p2p/nat: request timeouts for UPnP discovery

This includes a fix adding a timeout to router discovery requests.

tests: log coalescing fixes

cmd: transaction pool fixes and improvements 

eth/downloader: fix #1178, don't request blocks beyond the cache bounds