By Deepa Seetharaman SAN FRANCISCO (Reuters) – Amazon.com Incs quarterly results on Thursday may heighten worries that its edge over online and even traditional brick-and-mortar rivals is narrowing as it charges sales tax in more states, curtailing growth in its core business. Amazon, which has expanded aggressively into mobile devices and computing services to try to sustain its pace of growth, may lose customers to a growing crop of retail startups and major chains that had been slow to react but are now focusing on their own online commerce strategies, analysts say. …