Data: SDS no longer extends DataStructureImpl to save space

core/java/src/net/i2p/data/KeyCertificate.java

core/java/src/net/i2p/data/PrivateKeyFile.java

core/java/src/net/i2p/data/SimpleDataStructure.java

core/java/src/net/i2p/data/i2cp/CreateSessionMessage.java

core/java/src/net/i2p/data/i2cp/DestLookupMessage.java

...java/src/net/i2p/data/i2cp/ReconfigureSessionMessage.java