fix: do not block event loop while doing cleanups

[simllll]

potenitally fixes #215, needs more testing though

note: I have no idea of coffeescript, is my fix syntacitally correct?

[simllll]

Oh right, some test fail now due to the async behaviour. Either we await the setImmediate callback (Can we use async / await in coffeescript?) or we change some other code logic here to let this thing happen asynchronously.

[simllll]

Javascript code that solves the issue on my side:

async _checkData(startPeriod = true) {
        var key, ref, value;
        boundMethodCheck(this, NodeCache);
        ref = this.data;
        // run the housekeeping method
				var i = 0;
        for (key in ref) {
          value = ref[key];
					this._check(key, value);
					if (i % 10 === 0) {
						await new Promise(resolve => setImmediate(resolve));
					}
					i++;
        }
....

[stale]

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