fix(cpuUsageThrottle): support breaking change in pidusage module

[retrohacker]

A commit semver major upgraded the pidusage module.

https://github.com/restify/node-restify/pull/1755/files#diff-b9cfc7f2cdf78a7f4b91a753d10865a2R109

The cpuUsageThrottle tests proxyquired out the entire module, preventing them from failing even though the module changed it's API.

node-restify/test/plugins/cpuUsageThrottle.test.js

Lines 12 to 18 in e4007cf

	var cpuUsageThrottle = proxyquire('../../lib/plugins/cpuUsageThrottle.js', {
	pidusage: {
	stat: function(pid, cb) {
	return cb(null, { cpu: CPU });
	}
	}
	});

This PR:

• Updates the plugin to use the new API for the pidusage module
• Adds a test which validates the API for the pidusage module

[rajatkumar]

Thanks for identifying this!