OAK-1282: Clean up RepositoryStub classes
- remove redundant OakRepositoryStub and OakRepositoryStubBase (use OakTarMKRepositoryStub instead)
- add a repositoryHelperPool.properties config to limit the pool to one stub instance
- switch to OakTarMKRepositoryStub in repositoryStubImpl.properties

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1550453 13f79535-47bb-0310-9956-ffa450edef68