By George Georgiopoulos ATHENS (Reuters) - Greece's international lenders have asked Athens to halt National Bank's takeover of rival Eurobank, worried that the resulting lender would be too big for the state to deal with, two bankers close to the talks told Reuters on Saturday. The lenders' concerns come as an unexpected twist in the takeover deal, which was launched in October and completed with a share swap. The two banks have already begun integrating operations after getting approval by authorities. National Bank, Greece's biggest lender, took over 84. ...