SessionInternal::clear() erroneously calls start_session() when session is still open

[rotdrop]

SessionInternal::clear() does strange things:

server/lib/private/Session/Internal.php

Lines 103 to 108 in f5d1365

	public function clear() {
	$this->invoke('session_unset');
	$this->regenerateId();
	$this->startSession(true);
	$_SESSION = [];
	}

Error: session_start(): Ignoring session_start() because a session is already active at /var/www/localhost/htdocs/nextcloud-git/lib/private/Session/Internal.php#208 

1. session_unset() -- ok, destroys all values
2. regenerateId() -- ok, generate a new session id
3. start_session() -- oops. But the session is still open.

So either the start_session is superfluous and can be omitted -- at least in current versions of php -- or there is missing something like a session_destroy() (but then regenerate_id() could be omitted?) or a session_write_close() which would write the (now empty) session file to disk.