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.

i would prefer to have it call SetTree... then it's generic and could be used for other things (e.g. user mgt)