SLING-3384 Reimplement SlingServerRepository

Move repository creation out into a new SlingServerRepositoryManager class
(extending AbstractSlingRepositoryManager) and the old SlingServerRepository
class just implementing the createAdministartiveSession method.

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1566551 13f79535-47bb-0310-9956-ffa450edef68