Currently the mempool isn't persisted across restarts. The historical argument against doing this is that it would make transactions immortal. Expiration now addresses that issue.

Without preserving the mempool node behavior changes drastically across restarts which complicates production use (e.g. mining overly small/low fee blocks) as well as development testing. The lack of retention also causes the loss of txn priority modifications made with prioritisetransaction which makes the API less reliable.

if (pwalletMain)
        pwalletMain->Flush(true);
#endif
#if ENABLE_ZMQ
    if (pzmqNotificationInterface) {
        UnregisterValidationInterface(pzmqNotificationInterface);
        delete pzmqNotificationInterface;
        pzmqNotificationInterface = NULL;
    }
#endif
#ifndef WIN32
    try {
        boost::filesystem::remove(GetPidFile());
    } catch (const boost::filesystem::filesystem_error& e) {
        LogPrintf("%s: Unable to remove pidfile: %s\n", __func__, e.what());
    }
#endif
    UnregisterAllValidationInterfaces();
#ifdef ENABLE_WALLET
    delete pwalletMain;
    pwalletMain = NULL;
#endif
    globalVerifyHandle.reset();
    ECC_Stop();
    LogPrintf("%s: done\n", __func__);
}
/**
 * Signal handlers are very limited in what they are allowed to do, so:
 */
void HandleSIGTERM(int)