OPENDJ-1628 - Simplify Index hierarchy and remove Indexer classes

* DatabaseContainer / Index
    - now interfaces providing minimal set of methods
* NullIndex
    - implements Index interface and forces us to override new interface methods when they are added
* AbstractDatabaseContainer
    - common base class for all indexes (was DatabaseContainer)
* DefaultIndex
    - implementation of id2children, id2subtree, and base implementation for attribute indexes
* AttributeIndex
    - includes MatchingRuleIndex inner class which is used for all matching rule based indexes and presence indexing (presence is now implemented using a specialized Indexer)
    - pushed down all attribute index specific logic from Index API to AttributeIndex/MatchingRuleIndex
* IndexBuffer
    - remove feature envy between this class and Index
    - used consistent method/field/paramet names throughout.