In some cases, it might be desirable to clear cached chat messages once you've
reconnected to the XMPP server.
For example, if you want to prevent the chat history from getting too long or
if you want to avoid gaps in the chat history (for example due to MAM not
returning all messages since the last cached message).
If you're using OMEMO, then you probably don't want to set this setting to
``true``. OMEMO messages can be decrypted only once, so if they then
subsequently get cleared, you won't get the plaintext back.
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