Expand extranonce2 size to 64-bit

It's rather unlikely to overflow a 32-bit extranonce2 (requires over 4 Gh performed on CPU; with a 1 minute time between jobs, that's over 50 Mh/s), but it doesn't hurt much
Failover pool switching back delay is now configurable.

--failover-switch-delay sets the delay in seconds before switching back to a failed pool (default is 300 seconds).

Conflicts:
	miner.c
Make pool stability message use nicer English