les: les/4 minimalistic version (#21909)

* les: allow tx unindexing in les/4 light server mode

* les: minor fixes

* les: more small fixes

* les: add meaningful constants for recentTxIndex handshake field

les/protocol.go

@@ -50,6 +50,11 @@ var ProtocolLengths = map[uint]uint64{lpv2: 22, lpv3: 24, lpv4: 24}
 const (
 	NetworkId          = 1
 	ProtocolMaxMsgSize = 10 * 1024 * 1024 // Maximum cap on the size of a protocol message
+	blockSafetyMargin  = 4                // safety margin applied to block ranges specified relative to head block
+
+	txIndexUnlimited    = 0 // this value in the "recentTxLookup" handshake field means the entire tx index history is served
+	txIndexDisabled     = 1 // this value means tx index is not served at all
+	txIndexRecentOffset = 1 // txIndexRecentOffset + N in the handshake field means then tx index of the last N blocks is supported
 )
 
 // les protocol message codes