cpuUsageThrottle plugin `close()` does not clean up properly

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Bug Report

When cleaning up resources on shutdown (or test teardown), calling close() on the plugin does not properly clean up all resources.

Restify Version

8.3.2

Node.js Version

All

Expected behaviour

Calling plugin.close() should allow process to exit.

Actual behaviour

close() does not clean up handles, leaving process running.

Repro case

const cpuUsageThrottle = require('restify').plugins.cpuUsageThrottle;

const plugin = cpuUsageThrottle({
    limit: 0.75,
    max: 1,
    interval: 50,
    halfLife: 200
});

plugin.close();

Cause

It seems there are two causes:

1. pidusage.clear() needs to be called, as pidusage itself attaches a setInterval on the process.
2. There is race condition where plugin.close() can be called before pidusage()'s first invocation can return. In that scenario, plugin._timeout is null as it has never been initialized, and therefore the re-looping setTimeout never gets unregistered.

Start with the cpuUsageThrottle plugin and its close() path; inspect the pidusage cleanup and the plugin._timeout race described in the report. Reproduce the supplied snippet, then verify that close() releases all handles and allows the process to exit.