By Chris Mannix Claressa Shields could be primed to face longtime rival Savannah Marshall next month after last week’s fight was postponed due to Queen Elizabeth’s death. LONDON – Representatives for Claressa Shields are optimistic that Shields will be available to face longtime rival Savannah Marshall on Oct. 15 in the United Kingdom, Shields’s manager, Mark Taffet, told Sports Illustrated on Monday. Shields-Marshall, originally scheduled for Sept. 10, was postponed in the aftermath of Queen Elizabeth’s death last week. In announcing the postponement, Boxxer, the lead promoter of the historic…