eth/protocols/eth: increase maxTxPacketSize from 100 KB to 1 MB for Polygon throughput

Polygon mainnet processes 120M gas/block at 2s block times. At high
utilization (90-100% gas), blocks carry 430-520 KB of transaction data,
with full blocks reaching ~520 KB. The previous 100 KB limit required
5+ sequential p2p messages per peer per full block.

This affected both paths:
- Direct broadcast (full txs to sqrt(peers)): many sequential messages
- GetPooledTransactions responses: peers fetching txs after announcement
  could not receive a full block's worth in one response, increasing
  round trips for mempool convergence across the wider peer set

1 MB is chosen to:
- Fit any block at current gas limits in a single message
- Provide ~2x headroom over the current full-block maximum (~520 KB),
  absorbing a gas limit increase to ~200M without another bump
- Stay at 6.3% of the 16 MB p2p message size limit
- Remain well below the ~15 MB witness messages already exchanged on
  this network, keeping head-of-line blocking risk negligible

Note: 512 KB was considered but rejected — full blocks already reach
520 KB, leaving essentially no margin.

Author: lucca30

eth/protocols/eth/broadcast.go

@@ -24,9 +24,13 @@ import (
 )
 
 const (
-	// This is the target size for the packs of transactions or announcements. A
-	// pack can get larger than this if a single transactions exceeds this size.
-	maxTxPacketSize = 100 * 1024
+	// maxTxPacketSize is the target size for packs of transactions or announcements.
+	// A pack can get larger than this if a single transaction exceeds this size.
+	//
+	// Increased from the go-ethereum default of 100 KB to 1 MB: at Polygon's
+	// 120M gas/block, a single full block carries ~520 KB of transaction data,
+	// so the default would require 5+ sequential packets per block per peer.
+	maxTxPacketSize = 1024 * 1024
 )
 
 // blockPropagation is a block propagation event, waiting for its turn in the