code: void purgeDataOlderThan(long minTxIdToKeep) throws IOException {
      if (isImageDir) {
        purgeMatching(sd.getCurrentDir(), IMAGE_CURRENT_DIR_PURGE_REGEXES,
            minTxIdToKeep);
      } else {
        purgeMatching(sd.getCurrentDir(), EDITS_CURRENT_DIR_PURGE_REGEXES,
            minTxIdToKeep);
        purgeMatching(getPaxosDir(), PAXOS_DIR_PURGE_REGEXES, minTxIdToKeep);
      }
    }
docs: Remove any log files and associated paxos files which are older than
  the given txid.