Move Hawk storage to per-plugin state locations

We have observed that the Hawk Docker image does not work properly with existing storage volumes across regular updates (those that would not require a reindex). This may be due to how the Hawk Docker image uses a storage volume for the OSGi configuration area, in order to preserve Hawk index configurations (which are usually kept there).

After further research, I found that we should not have been using the OSGi configuration area for storage in the Thrift-based server: instead, we should have been using the plugins' "state location" folders, which are located in the workspace. This way, the Docker image would only need two storage volumes: one for the keyring, and one for the server's workspace.