DS-1990: Events should contain all identifiers, not only handles.
Fixes typo in a javadoc comment.
Bitstreams should set identfiers to Events as well.
Enhancing the PR for DS-1990.

Adds a cache to the Identifier array in the DSpaceObject class.
Renames the method from lookupIdentifiers into getIdentifiers.
DS-2125 Fix possible NPEs in Context object commit/complete/abort. Also add Unit Tests for Context object
DS-2127: Upgrade commons-codec to version 1.9.
Asynchronous IdentifierProvider should call DSO.resetIdentifierCache.

Adds a note to the javadoc of DOIIdentifierProvider to reset the
identifiers cache in case of asynchronous identifier registration.
Adds a call to reset identifiers cache to DOIIdentifierProvider which
uses asynchronous registration.
DSpaceObject.getIdentifier(Context) should never return null.

If an DSO does not have any identifiers (including handle) this method
should return an empty string array. G.e. an item during submission
(workspace item) has no identifiers at all. This commits prevents NPE in
event handling classes.
Add logging for abort() and complete() when they are called on an invalid/closed Context
Merge pull request #627 from tuub/DS-2127

DS-2127: Upgrade commons-codec to version 1.9.
Merge pull request #537 from tuub/DS-1990

DS-1990: Events should contain all identifiers, not only handles.
Merge pull request #626 from tdonohue/DS-2125

DS-2125 Fix possible NPEs in Context object commit/complete/abort