“The apparent conflict between the AAA Class Rules and the FLSA Section 16(b) provision would necessarily be resolved in favor of an ‘opt-in’ procedure if the consent requirement of Section 16(b) is a substantive right, not waivable by an arbitration agreement.” Although the defendant’s references to the text and legislative history of the FLSA “reassure us of Congress’s intention that the ‘opt-in’ procedure should apply in arbitration as in court proceedings, they fail to also convince us that Congress expressly intended that the ‘opt-in’ procedure could not be waived by the parties’ agreement to an alternate procedure.” See Long John Silver v. Cole, 513 F.3d 345 (4th Cir. 2008).