Avoid unnecessary reply ABORT in kafka merged stream

[jfallows]

-> END kafka merged (initial)
  -> END describe (initial)
  <- END describe (reply)
<- END kafka merged (reply) attempted but no BEGIN, so no-op
  -> ABORT unmerged fetch (initial)
  <- ABORT unmerged fetch (reply)
<- BEGIN kafka merged (reply)
<- ABORT kafka merged (reply)