AutoRefactor'ed the pluggable and PDB backend

OPENDJ-1849 CR-6871 Remove references to JE from pluggable backend Additional decoupling of pluggable backend from JE. Class renames, comments and javadoc line up with new terminology, and general cleanup. Renamed classes: AbstractDatabaseContainer to AbstractTree DatabaseContainer to Tree DatabaseEnvironmentMonitor to BackendMonitor (given what the code does) DbPreloadComparator to TreePreloadComparator