Add websocket cleanup policy

[frankier]

The idea here is that

1. People can customize the websocket cleanup behavior in Bonito
2. Those wanting to manage their own websockets can make use of the same pluggable behaviour including using the default policy from Bonito.

Builds on https://github.com/SimonDanisch/Bonito.jl/pull/247

[SimonDanisch]

Thank you, this looks good, besides my comment ;) We may want to have more tests for this as well.

[frankier]

Done and done

[frankier]

@SimonDanisch Ping. Ready to merge?

[SimonDanisch]

Thank you, looks good :)