OPENDJ-1602 (CR-5566) New pluggable storage based backend

Replacing calls to BackendImpl.Storage.openCursor() by calls to ReadOperation.openCursor().
It required to pass ReadOperation down many method calls.
In several applyConfiguration*() methods, created write operations.
In several applyConfiguration*() methods, created a ConfigChangeResult object at the start of the method and accumulated information on it instead of creating it at the end of the method + passed the ConfigChangeResult object down method calls.

DatabaseContainer.java:
In open(), simplified the code.

EntryContainer.java:
Removed useless comments.

ConfigChangeResult.java:
Added default ctor.
Code cleanup.