fix: fixed memory leak and potential use-after-free bug with voiceconn

[Mishura4]

dpp::discord_voice_client does not free its active voice connections on destruction. This means that, when the cluster/voice client gets destroyed, any dpp::voiceconn objects as well as threads they have running won't be joined or freed. This causes not only a memory leak, but in the event of the cluster/voice client getting destroyed, and the still alive voiceconn thread trying to for example log a message, a use-after-free will occur and cause undefined behavior (usually a crash). This has happened to me about on about 50% of my executions of the unit tests on Windows.

This PR fixes this bug. However it does make use of std::unique_ptr, that as I understand it the library does not usually use, let me know if you would like me to change it to new/delete instead.

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[braindigitalis]

is the pointer now freed by removal of the iterator from the map?

[Mishura4]

Pretty much, or when the map gets destroyed. The map owns the pointer, and when one of its elements is destroyed, the voiceconn is also destroyed and freed.

I didn't find any other code that made use of that map in a modifying way e.g. grabbed/changed the pointer, so this change should be fine.