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.

well... IMO that doesn't work with a static list of operations... the way you use it currently:
- you first add the sub-operations to the static field 
- then you create the SetPolicy object

IMO this should be as follows:
- first create a new instance of SetPolicy
- then add the sub-operations to that instance