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