feat: allow replacement of oldest key

[doc-han]

Feature

This pull request is to allow the auto replacement of the oldest key when trying to do a set operation and the max amount of keys has been reached.
Requested by #232

Example

const nodeCache = require("node-cache");
const myCache = new nodeCache({maxKeys: 2, replaceOldestKey: true});

myCache.set("one", "This is the first key");
myCache.set("two", "This is the second key");
// from here maxKeys has been reached, hence, the oldest key which is "one" gets removed
myCache.set("three", "This is the third key");
console.log(myCache.keys());
// output: ["two", "three"]

[cekvenich2]

Is there a fork where this is included?

[doc-han]

#212 will make the code on this fix better. Since a Map keeps the order of insertion it'll be easier to locate the oldest key.

[Recodify]

Would really love this to be merged! Really useful for allowing you to set some sort of vague limit on the size the cache is allow to grow to without preventing new items being inserted. A MAX_HEAP_SIZE option that utilised this "allow replacement of oldest key" method on new insertions would be the holy grail