pd: pick a smaller value for mempool.max_tx_bytes

[dynst]

In the cometbft config template, maximum individual transaction size (max_tx_bytes) should probably be smaller than the maximum block size (the consensus parameter) that just got lowered in #4597. What's a reasonable value?

[erwanor]

We need empirical data to set a judicious default in the template, so we can revisit later, but a safe upper bound should be ~30KB. This gives ample room for a user to open a collection of replicating LPs and roll block-scoped orders.