By Antony Currie The author is a Reuters Breakingviews columnist. The opinions expressed are his own. Jamie Dimon’s management shake-up doesn’t just send a message to the bank’s employees and shareholders. The timing of the move – and the language in the announcement – serves as the JPMorgan chief executive’s response to the reignited debate over the structure of big financial institutions like his. Dimon lauded the idea of knitting JPMorgan’s parts together more tightly just as his former mentor Sandy Weill converted to the breakup cause.