[processor/memorylimiter] Check the pipeline to differentiate between refused and dropped

[iblancasa]

          tangent: I wonder whether this comment (which appears 4 times now) will ever be resolved.

Originally posted by @jmacd in #12454 (comment)

[dmathieu]

Original TODO:

// TODO: actually to be 100% sure that this is "refused" and not "dropped"
// it is necessary to check the pipeline to see if this is directly connected
// to a receiver (ie.: a receiver is on the call stack). For now it
// assumes that the pipeline is properly configured and a receiver is on the
// callstack and that the receiver will correctly retry the refused data again.