Do not recommend "X-" prefixed headers

Quote from PR https://github.com/jbosstm/microprofile-sandbox/pull/3 raised by @nicolaferraro :

A small change to the spec in order to use "Long-Running-Action" and "Short-Running-Action" headers instead of "X-lra" and "X-sra".
"X-" prefixed headers should be deprecated by RFC-6648 and RFC-7231.

They are not mandated by this spec, but they'll be used by microprofile rest services, so it's important for other technologies to use them if they want to build interoperable rest services.