Add in-memory queue option

[ninjabear]

As part of a tuned-for-throughput version of the tracker, avoiding disk accesses unlocks some extra potential.

We can achieve this by implementing the IStorage interface (this can be done dynamically). We should provide an option for this out of the box.

It'd look like this:

using Snowplow.Tracker.Storage;
using System;
using System.Collections.Generic;
using System.Linq;

...

public class MemoryStorage : IStorage
    {
        private List<string> _items = new List<string>(1000);

        public int TotalItems => _items.Count;

        public bool Delete(List<string> idList)
        {
            var ids = (from i in idList select int.Parse(i)).OrderByDescending(x=>x).ToList();
            ids.ForEach(id => _items.RemoveAt(id));
            return true;
        }

        public void Put(string item)
        {
            _items.Insert(0, item);
        }

        public List<StorageRecord> TakeLast(int n)
        {
            int totalBack = Math.Min(_items.Count(), n);
            int startIndex = _items.Count() - totalBack;

            var last = new List<StorageRecord>(totalBack);

            for (int idx=startIndex; idx<_items.Count(); idx++)
            {
                var item = new StorageRecord { Id = idx.ToString(), Item = _items[idx] };
                last.Add(item);
            }

            return last;
        }
    }

[paulboocock]

Switch to making the Queue in memory, as there is no need to use the "PersistentQueue" concept when building an in memory buffer option. Also much easier to make this thread safe.