Updated IPv8 exit node script to use RustEndpoint

[egbertbouman]

This PR does the following:

• Enables RustEndpoint in the IPv8 exit node script. I haven't enabled it for all IPv8 nodes, as I don't think it makes sense since normal nodes will be making calls from within Python anyway (removing any chance of a performance improvement).
• Add the ability for the TunnelCommunity to deal with DispatcherEndpoints.
• Fixes a potential KeyError in relays_cell for rendezvous points.
• Fixes an issue with leftover asyncio tasks after TunnelExitSocket removal.

I also fixed some issues with the CommunitySettings:

• Fixes an issue with incorrect peer flags during the tunnel tests.
• Fixes an issue with properties in CommunitySettings.
• Fixes an issue with the default tunnel settings getting stored in overlay.settings.settings instead of in overlay.settings.

[egbertbouman]

It looks like in cryptography 42.0 (released less than a day ago) the functions BN_clear_free/BN_clear/BN_zero aren't available anymore. So, I've removed BN_clear_free in favor of manually setting the BIGNUM to 0, followed by a call to BN_free. Not sure this is correct, but given that attestion logic isn't used anyway this seems OK.