Add FeatureDefinitionReference for serializing fd references

Add the FeatureDefinitionReference class which allows to create
serializable references to feature holders.
Fix DefaultFunction test case names

Rename the AbstractFunctionXXXTest cases to DefaultFunctionXXXTest.
Use apache commons lang MutableXX objects instead of AtomicXX

Replace usages of AtomicXX objects for final field modifiation with the
MutableXX classes from apache commons lang.
Update changelog