JCR-3678: MembershipCache max size is hard coded to 5000

Allow the cache size to be controlled with a system property.
We can turn this into a proper repository configuration parameter
if the need to change the cache size is a common enough.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/trunk@1530005 13f79535-47bb-0310-9956-ffa450edef68