private File decompressApplication(CompressedApplicationInputStream in, File tempDir) {
        try {
            return in.decompress(tempDir);
        } catch (IOException | UncheckedIOException e) {
            throw new IllegalArgumentException("Unable to decompress application stream", e);
        }
    }
Gets the active Session for the given application id.

@return the active session, or null if there is no active session for the given application id.
configserver/src/main/java/com/yahoo/vespa/config/server/ApplicationRepository.java