Memory leak in SynchronizedConverter

[Airex]

in case of using of SynchronizedConverter PdfTools is as singleton and contains _Delegates member that is never cleared and list just fills with delegates on each conversion

_Delegates.Clear(); in DestroyConverter should remove this issue

[xamele0n]

I purpose to change "delegates" to

 //used to maintain a reference to delegates to prevent them being garbage collected...
private readonly ConcurrentDictionary<IntPtr, List<object>> delegates = new ConcurrentDictionary<IntPtr, List<object>>();

and use

        public void DestroyConverter(IntPtr converter)
        {
            this.delegates.TryRemove(converter, out var l);
            l?.Clear();
            this.module.wkhtmltopdf_destroy_converter(converter);
        }

to free associated delegates

[HakanL]

Please submit PR for review

[HakanL]

I've implemented this now in version 1.3.0, please verify.