Use client versioning for private groups.

...briar/privategroup/invitation/AbstractProtocolEngine.java

.../briar/privategroup/invitation/CreatorProtocolEngine.java

...arproject/briar/privategroup/invitation/CreatorState.java

...iar/privategroup/invitation/GroupInvitationConstants.java

...r/privategroup/invitation/GroupInvitationManagerImpl.java

.../briar/privategroup/invitation/GroupInvitationModule.java

.../briar/privategroup/invitation/InviteeProtocolEngine.java

...arproject/briar/privategroup/invitation/InviteeState.java

...ect/briar/privategroup/invitation/PeerProtocolEngine.java

...briarproject/briar/privategroup/invitation/PeerState.java

...ar/privategroup/invitation/ProtocolEngineFactoryImpl.java

...ect/briar/privategroup/invitation/SessionEncoderImpl.java

...rproject/briar/privategroup/invitation/SessionParser.java

...ject/briar/privategroup/invitation/SessionParserImpl.java

...org/briarproject/briar/privategroup/invitation/State.java

...r/privategroup/invitation/AbstractProtocolEngineTest.java

...ar/privategroup/invitation/CreatorProtocolEngineTest.java

...ivategroup/invitation/GroupInvitationManagerImplTest.java

...ar/privategroup/invitation/InviteeProtocolEngineTest.java

...briar/privategroup/invitation/PeerProtocolEngineTest.java