Add sync hash table to control episode download cache

[SergioEstevao]

| 📘 Part of: # |
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Fixes #

Adds a synced hash table that is controlled by a synchronous queue and then use this to controls access the episode downloads queue.

To test

1. Start the App
2. Choose a Podcast with multiple episodes
3. Select multiple episodes and start downloading them
4. Select another podcast and also start the downloads for it
5. Play an episode at the same time.
6. Check if no crash happens.

Checklist

• I have considered if this change warrants user-facing release notes and have added them to CHANGELOG.md if necessary.
• I have considered adding unit tests for my changes.
• I have updated (or requested that someone edit) the spreadsheet to reflect any new or changed analytics.

[danielebogo]

Just left few comments. Can you add few tests to make sure it works?

[danielebogo]

@SergioEstevao can you also update the Changelog?

[danielebogo]

works really good!

[danielebogo]

Thanks for this!