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

Removed the managed tx state from the chain manager to the transaction
pool where it's much easier to keep track of nonces (and manage them).
The transaction pool now also uses the queue and pending txs differently
where queued txs are now moved over to the pending queue (i.e. txs ready
for processing and propagation).

The transaction pool drops processed transactions on its own
during pool maintenance.