The "if" interceptor supports conditional execution of a group of executors.

Note that this is a draft implementation only: Design decisions are still pending.

• 'evaluate condition only once': Should the condition be reevaluated once response handling has begun?
• 'evaluate condition always during request handling already' (even when 'if' is nested in 'response')
• What happens to ABORT handling of interceptor A in <request><if test="..."><A /></if></response>