static createBuckets_(sessions) {
    const SessionDeleter = shaka.offline.SessionDeleter;

    /** @type {!Array<shaka.offline.SessionDeleter.Bucket_>} */
    const ret = [];
    for (const session of sessions) {
      let found = false;
      for (const bucket of ret) {
        if (SessionDeleter.isCompatible_(bucket.info, session)) {
          bucket.sessionIds.push(session.sessionId);
          found = true;
          break;
        }
      }
      if (!found) {
        ret.push({info: session, sessionIds: [session.sessionId]});
      }
    }

    return ret;
  }
Collects the given sessions into buckets that can be done at the same time.
Since querying with different parameters can give us back different CDMs,
we can't just use one CDM instance to delete everything.

@param {!Array<shaka.extern.EmeSessionDB>} sessions
@return {!Array<shaka.offline.SessionDeleter.Bucket_>}
@private