[server] Only register "clean_sessions" looping tasks if repository has a tasks manager This prepares for calling Repository's _prepare_startup() in bootstrap() so that repository instances without a tasks manager running can still have their 'server_startup' hooks triggered (this is particularly useful for BFSS storages).

[utils] Do not pass a tasks manager to Repository in admincnx() We do not call repo.start_looping_tasks() for the instantiated repository, so a tasks manager is useless.

[server] Separate repository bootstrap from initialization Thus we now call 'bootstrap' here and there explicitly and remove call of this method in Repository.__init__(). This way instantiation of a Repository does not *implicitly* triggers the "bootstrap" step, which is arguably not a trivial thing and thus deserves to be independent. In __init__, set 'shutting_down' attribute to None and then to False in bootstrap as a mean to indicate that, when initialized, a repository is neither shutting down nor started (not sure where this is used though).