The man who oversaw more than $45 million in upgrades to the port of Portland and helped transform it into a modern shipping terminal has resigned. John Henshaw, executive director of the Maine Port Authority, stepped down Wednesday after a decade of leading the state agency. During his tenure, Henshaw oversaw the expansion of the International Marine Terminal in Portland and is credited with helping convince Eimskip, an Icelandic shipping company, to locate its North American headquarters in Maine.