perf: Skip redundant __init__ assignments and remove dead attributes in ResponseFuture (-32ns/call - 4.7% improvement for the common case, +43ns (+4.5%) for the non-common case, - code cleanup also)

perf: Skip redundant __init__ assignments and remove dead attributes in ResponseFuture (-32ns/call - 4.7% improvement for the common case, +43ns (+4.5%) for the non-common case, - code cleanup also) #721