Faster tear down of tx-submission

[karknu]

If the server wants to be able to exit it should stop providing clients with new txids and it should omit tx-bodies in replies to tx requests.

[coot]

The outbound side is non-piplined, it simply sends MsgDone when it's requested to stop:

ouroboros-network/ouroboros-network/lib/Ouroboros/Network/TxSubmission/Outbound.hs

Lines 137 to 145 in 42e7a5a

	mbtxs <- timeoutWithControlMessage controlMessageSTM $
	do
	MempoolSnapshot{mempoolTxIdsAfter} <- mempoolGetSnapshot
	let txs = mempoolTxIdsAfter lastIdx
	check (not $ null txs)
	pure (take (fromIntegral reqNo) txs)
	case mbtxs of
	Nothing -> pure (SendMsgDone ())