JCR-2113: Writing access control

This patch addresses the following as indicated in https://issues.apache.org/jira/browse/JCR-2113:
a. adjust jcr2dav module and make sure the RepositoryConfig gets a valid ac-provider configuration parameter set
b. complete setPolicy, Batch#setTree, + testing
c. complete removePolicy for jcr2spi and complete implementation for removal of protected items on jcr-server.

in general i am not too convinced that having TreeImpl and PropertyImpl in spi.commons is really a good idea... i would rather extend the Tree interface with the ability to add property and child node information and leave the individual handling with the various batch implementations instead of having to convert them twice... will provide an example.