mixpool: Limit caching of KE messages

[jholdstock]

Every message type apart from KE goes through the following check in AcceptMessage to ensure only one message of each type per identity per session:

dcrd/mixing/mixpool/mixpool.go

Lines 1302 to 1315 in 3836103

	// Check that a message from this identity does not conflict with a
	// different message of the same type in the session.
	var haveKE bool
	for _, prevHash := range p.messagesByIdentity[*id] {
	e := p.pool[prevHash]
	if e.msgtype == msgtype && bytes.Equal(e.msg.Sid(), msg.Sid()) {
	return nil, ruleError(fmt.Errorf("message %v by identity %x "+
	"in session %x conflicts with already accepted message %v",
	hash, *id, msg.Sid(), prevHash))
	}
	if !haveKE && e.msgtype == msgtypeKE && bytes.Equal(e.msg.Sid(), msg.Sid()) {
	haveKE = true
	}
	}

Not applying this check to KE messages means there is no upper bound on how many KE messages can be sent by a single peer and an unlimited number of these messages will be cached in memory.

[jholdstock]

Related: KE messages are rejected if their epoch is too far in the future but not if their epoch is in the past.