This change cancels and joins the family DecoratorManager.FAMILY_DECORATE
after a Java editor is opened in SaveParticipantTest.

The job is occasionally seen in debug traces, adding element infos
to JavaModelManager.cache. Additionally it can open and close buffers at
org.eclipse.jdt.internal.core.BufferManager, where buffers
from the BufferManager are used for retrieving the contents of files
that should be formatted on save.

The change also adds debug traces for BufferManager.

See: #79