server_side: plugin refactoring: twisted.plugins.libervia_server is refactored for the same reason as for SàT backend (see SàT commit message for revision adea30ca0b51), and there is an additionnal trick: as we need to use some variables in both twisted.plugins.libervia_server and server.server, we can't import server.server before initialise() is called, and we can't neigher import twisted.plugins.libervia_server from server.server (there is no __init__.py file as requested by Twisted plugin systeme), these variable have been moved from server.server to twisted.plugins.libervia_server, and are set in server.server from the latter. This not super clean to read, but it solve the import order issues.