node.Node no longer registers any account manager backends by default,
they need to be registered explicitly.
For ethash-based tests, we actually don't need any accounts in the miner
keystore. Just set the etherbase instead to make mining work. For
clique, the signer account must be in the keystore.
The change also adds interrupt handling in stress tests.
Péter Szilágyi
Håvard Anda Estensen
Felix Lange
gary rong