mqb: fix ClientSession::initiateShutdown crash

[dorjesinpo]

    // The 'd_self.acquire()' return 'shared_ptr<ClientSession>' but that does
    // not relate to the 'shared_ptr<mqbnet::Session>' acquired by
    // 'mqbnet::TransportManagerIterator'.  The latter is bound to the
    // 'initiateShutdown'.  The former can be null after 'd_self.invalidate()'
    // call. ('invalidate()' waits for all _acquired_ 'shared_ptr' references
    // to drop).
    //
    // We have a choice, either 1) bind the latter to 'initiateShutdown' to
    // make sure 'd_self.acquire()' returns not null, or 2) invoke the
    // 'callback' earlier if fail to 'acquire()' because of 'invalidate()', or
    // 3) bind _acquired_ 'shared_ptr' to 'initiateShutdown'.
    //
    // Choosing 2), assuming that calling the (completion) callback from a
    // thread other than the *CLIENT* dispatcher thread is ok.  The
    // 'mwcu::OperationChainLink' expects the completion callback from multiple
    // sessions anyway.