Cleanup session and state handling

.../justjanne/libquassel/client/ClientProxyMessageHandler.kt

...kotlin/de/justjanne/libquassel/client/ClientRpcHandler.kt

...in/kotlin/de/justjanne/libquassel/client/ClientSession.kt

...lin/de/justjanne/libquassel/client/io/CoroutineChannel.kt

...e/justjanne/libquassel/client/io/CoroutineChannelState.kt

...janne/libquassel/client/syncables/ClientBacklogManager.kt

...tjanne/libquassel/client/syncables/ClientIrcListHelper.kt

.../justjanne/libquassel/protocol/session/CommonSyncProxy.kt

...ne/libquassel/protocol/session/HandshakeMessageHandler.kt

...tjanne/libquassel/protocol/session/ProxyMessageHandler.kt

...otlin/de/justjanne/libquassel/protocol/session/Session.kt

...lin/de/justjanne/libquassel/protocol/session/SyncProxy.kt

...e/libquassel/protocol/syncables/StatefulSyncableObject.kt

...justjanne/libquassel/protocol/syncables/SyncableObject.kt

...e/justjanne/libquassel/protocol/syncables/SyncableStub.kt

...anne/libquassel/protocol/syncables/common/AliasManager.kt

...ne/libquassel/protocol/syncables/common/BacklogManager.kt

...anne/libquassel/protocol/syncables/common/BufferSyncer.kt

.../libquassel/protocol/syncables/common/BufferViewConfig.kt

...libquassel/protocol/syncables/common/BufferViewManager.kt