Refactor DN and RDN classes and improve their test coverage.

The following refactoring work was performed:

* removed the DN, RDN, and AVA comparator classes. The DN
  and RDN classes implement Comparable. RDN.compareTo now
  uses ordering matching rules where possible (taken from
  the original RDN Comparator class)

* RDN class is now almost immutable (it's intention is to
  be 100% immutable). It is still possible to retrieve
  attribute values and modify them (attribute values should
  be immutable - issue raised)

* RDN compareTo, hashCode, and equals methods now
  sort AVAs based on the type's name/oid (previously no
  sorting was performed)

* provide factory methods for simple RDNs and a builder
  class for constructing more complex RDNs

* DN class is now immutable with the exception described above
  for RDN attribute values. There are now factory methods for
  creating, joining, and splitting DNs

* special factory method for creating null DNs - DN.nullDN()

* DN.getParent() renamed to DN.getParentDNInSuffix() - all references
  updated - this method will be moved in a subsequent change
  to the DirectoryServer class

* implemented a new DN.getParent() method which ignores suffixes

* DN no longer copy their RDN array when constructing parent
  DNs or localNames - instead array offset+length is identifies
  which part of the array is associated with the DN

* refactored and simplified much of the DN/RDN decoding routines