ObjectManager immediately methods and pending moves

[dbu]

cloneFrom and workspace copy method (ObjectManager::copyNodeImmediately) both check if a node already exists at $destAbsPath by going through the session, which will trigger getFetchPath. with jackrabbit storage, we will create same-name siblings under /b if i do this

$session->move('/b', '/y');
$workspace->cloneFrom('ws', '/x', '/b', false);
$session->save();

what would be allowed to create the sibling would be

$session->move('/b', '/a');
$workspace->cloneFrom('ws', '/x', '/a', false);
$session->save();

but this will currently say you can't clone as there is a target. this should clone but then session save would create /a[1] or throw an exception.

this needs a general solution, some sort of raw exists check without applying getFetchPath.

follow up from #155