MessageLogger: Fix message history loss on reconnect + unloaded channels

[reservedbytes]

Describe your Changes

I noticed that if someone sent me a message and then deleted it, the logger didn´t log it at all, after some reverse engineering (which also made me find another problem included in this PR which is losing messages after a reconnect), I found out that Discord only stores messages for channels that are ready.
Because my account is in ~150 servers, it was giving me an average of ~0.25 messages per second, to prevent the plugin from eating the memory very quickly with loads of unwanted modified/deleted messages, I made a simple memory filter for DMs and servers where you´ve opened any channel in the current session.

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