OCSP cache doesn’t have a means of clearing all cache invalidation check intervals

## Use case

While running unit tests that use the OCSP cache, the Node process does not exits due to active handles (the cache expiry intervals)

## Suggested fix

Add a method that clears the cache and cache expiry interval checks. e.g.,

```js
Cache.prototype.clear = function clear() {
var cacheIds = Object.keys(this.cache);
cacheIds.forEach(function (cacheId) {
clearInterval(this.cache[cacheId].timer);
})
this.cache.length = 0;
};
```

## Workaround

In your app, clear the intervals manually. e.g., in my app:

```js
const ocspCache = new ocsp.Cache()
// …
const cacheIds = Object.keys(ocspCache.cache)
cacheIds.forEach(cacheId => {
clearInterval(ocspCache.cache[cacheId].timer)
})
```

Locate the OCSP Cache implementation and inspect how cache entries store their expiry interval. Add a public clearing entry point that removes cached entries and stops every expiry check, then run the existing unit-test suite to confirm Node exits without active cache timers.

Written by the indexing model from the issue text.