Code cleanup.

BackendImpl.java:
Extracted methods removeDeletedBaseDNs(), createNewBaseDNs(), checkRootContainerInitialized(), mustOpenRootContainer().

git-svn-id: https://svn.forgerock.org/opendj/branches/opendj3-server-dev@11227 41b1ffd8-f28e-4786-ab96-9950f0a78031