mempool: disable MaxBatchBytes (#5800)

@p4u from vocdoni.io reported that the mempool might behave incorrectly under a
high load. The consequences can range from pauses between blocks to the peers
disconnecting from this node.

My current theory is that the flowrate lib we're using to control flow
(multiplex over a single TCP connection) was not designed w/ large blobs
(1MB batch of txs) in mind.

I've tried decreasing the Mempool reactor priority, but that did not
have any visible effect. What actually worked is adding a time.Sleep
into mempool.Reactor#broadcastTxRoutine after an each successful send ==
manual control flow of sort.

As a temporary remedy (until the mempool package
is refactored), the max-batch-bytes was disabled. Transactions will be sent
one by one without batching

Closes #5796

docs/tendermint-core/configuration.md

@@ -289,6 +289,7 @@ max_tx_bytes = 1048576
 
 # Maximum size of a batch of transactions to send to a peer
 # Including space needed by encoding (one varint per transaction).
+# XXX: Unused due to https://github.com/tendermint/tendermint/issues/5796
 max_batch_bytes = 10485760
 
 #######################################################