Implement persistent message storage

...chku/quasseldroid_ng/persistence/QuasselBacklogStorage.kt

...de/kuschku/quasseldroid_ng/persistence/QuasselDatabase.kt

...java/de/kuschku/quasseldroid_ng/service/QuasselService.kt

...a/de/kuschku/quasseldroid_ng/ui/chat/BufferListAdapter.kt

...n/java/de/kuschku/quasseldroid_ng/ui/chat/ChatActivity.kt

gradle/wrapper/gradle-wrapper.properties

lib/src/main/java/de/kuschku/libquassel/protocol/Message.kt

lib/src/main/java/de/kuschku/libquassel/protocol/QVariant.kt

...de/kuschku/libquassel/quassel/syncables/BacklogManager.kt

...ava/de/kuschku/libquassel/quassel/syncables/RpcHandler.kt

...main/java/de/kuschku/libquassel/session/BacklogStorage.kt

...main/java/de/kuschku/libquassel/session/CoreConnection.kt

lib/src/main/java/de/kuschku/libquassel/session/Session.kt

...main/java/de/kuschku/libquassel/session/SessionManager.kt