By Iain Withers and Saqib Iqbal Ahmed LONDON/NEW YORK (Reuters) – British bank Barclays said on Monday it had suspended the sales and issuance of two exchange-traded notes (ETNs) with combined assets of about $1 billion – one linked to crude oil and another to a gauge of market volatility – due to capacity constraints, in a move that some investors said could spur big price swings in the products. ETNs are debt securities that banks issue with the promise to pay holders a return linked to the performance of underlying securities or benchmarks. Investors are particularly sensitive about the pro…