whisper/whisperv6: implement pow/bloom exchange protocol (#15802)

This is the main feature of v6.

whisper/whisperv6/doc.go

@@ -35,7 +35,6 @@ import (
 )
 
 const (
-	EnvelopeVersion    = uint64(0)
 	ProtocolVersion    = uint64(6)
 	ProtocolVersionStr = "6.0"
 	ProtocolName       = "shh"
@@ -52,11 +51,14 @@ const (
 	paddingMask   = byte(3)
 	signatureFlag = byte(4)
 
-	TopicLength     = 4
-	signatureLength = 65
-	aesKeyLength    = 32
-	AESNonceLength  = 12
-	keyIdSize       = 32
+	TopicLength     = 4  // in bytes
+	signatureLength = 65 // in bytes
+	aesKeyLength    = 32 // in bytes
+	AESNonceLength  = 12 // in bytes
+	keyIdSize       = 32 // in bytes
+	bloomFilterSize = 64 // in bytes
+
+	EnvelopeHeaderLength = 20
 
 	MaxMessageSize        = uint32(10 * 1024 * 1024) // maximum accepted size of a message.
 	DefaultMaxMessageSize = uint32(1024 * 1024)
@@ -68,10 +70,8 @@ const (
 	expirationCycle   = time.Second
 	transmissionCycle = 300 * time.Millisecond
 
-	DefaultTTL     = 50 // seconds
-	SynchAllowance = 10 // seconds
-
-	EnvelopeHeaderLength = 20
+	DefaultTTL           = 50 // seconds
+	DefaultSyncAllowance = 10 // seconds
 )
 
 type unknownVersionError uint64