Calculation phi in out_forward.rb seems wrong.

The method 'phi(now) of 'FailureDetector class seems to calculate the variable 't wrong way.

When @heartbeat_interval is not the default value (1 sec), 't value contains (@heartbeat_interval - 1) sec. But 'mean doesn't. So @heartbeat_interval is set higher, 'phi(now) returns a higher value.

I'm sorry but I don't know the phi accuracy calculation logic precisely. But there seems no documentation or reference of the phi_threshold directive and this logic may confuse some people like me.

Thank you in advance.