The reactor is now set in the launcher, as it is the first script to be
loaded. This should avoid race conditions when some Twisted or libervia
packages are imported and end up importing `twisted.internet.reactor`,
which installs the default reactor before the AsyncIO one. The AsyncIO
reactor is still set in `twisted.plugin.libervia_backend_plugin` (but it
no longer fails if already set), in case the backend is started as a
plugin (although this is not recommended anyway)
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