I2CP: Don't have I2CP Messages extend DataStructureImpl, to save space

core/java/src/net/i2p/data/i2cp/I2CPMessage.java

core/java/src/net/i2p/data/i2cp/I2CPMessageImpl.java

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

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

core/java/test/junit/net/i2p/data/TunnelIdTest.java

...t/junit/net/i2p/data/i2cp/BandwidthLimitsMessageTest.java

...st/junit/net/i2p/data/i2cp/CreateLeaseSetMessageTest.java

...est/junit/net/i2p/data/i2cp/CreateSessionMessageTest.java

...a/test/junit/net/i2p/data/i2cp/DestLookupMessageTest.java

...va/test/junit/net/i2p/data/i2cp/DestReplyMessageTest.java

...st/junit/net/i2p/data/i2cp/DestroySessionMessageTest.java

...a/test/junit/net/i2p/data/i2cp/DisconnectMessageTest.java

...unit/net/i2p/data/i2cp/GetBandwidthLimitsMessageTest.java

...java/test/junit/net/i2p/data/i2cp/GetDateMessageTest.java

core/java/test/junit/net/i2p/data/i2cp/I2CPTstBase.java

core/java/test/junit/net/i2p/data/i2cp/MessageIdTest.java

...st/junit/net/i2p/data/i2cp/MessagePayloadMessageTest.java

...est/junit/net/i2p/data/i2cp/MessageStatusMessageTest.java

...nit/net/i2p/data/i2cp/ReceiveMessageBeginMessageTest.java

...junit/net/i2p/data/i2cp/ReceiveMessageEndMessageTest.java