OAK-1434: Cleanup dependencies of oak-core

Drop Oak(MicroKernel) since it's just a wrapper for Oak(new KernelNodeStore(MicroKernel)).
Also change the default Oak() constructor to use MemoryNodeStore instead of MicroKernelImpl.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1569690 13f79535-47bb-0310-9956-ffa450edef68