OpenVPN: Resolve issues with long negotiations (#1094)

Clean up the naive abuse of async/await in OpenVPNSession. Encapsulate
the instances of ControlChannel/DataChannel inside the Negotiator actor,
so that actor-isolation for them becomes automatically unnecessary.
Synchronous methods inside the actor are the way to go.

After that, handle control packets in orderly fashion, because this is
not being done and may spoil negotiation very easily. Probably also
happening in TunnelKit.

Ultimately, skip some unnecessary XOR processing in UDP when no XOR
method is actually set.

Packages/PassepartoutOpenVPNOpenSSL/Sources/PassepartoutOpenVPNOpenSSL/Internal/ControlChannel.swift

@@ -27,7 +27,7 @@ internal import CPassepartoutOpenVPNOpenSSL
 import Foundation
 import PassepartoutKit
 
-actor ControlChannel {
+final class ControlChannel {
     private let prng: PRNGProtocol
 
     private let serializer: ControlChannelSerializer
@@ -52,11 +52,11 @@ actor ControlChannel {
 
     private var sentDates: [UInt32: Date]
 
-    init(prng: PRNGProtocol) {
+    convenience init(prng: PRNGProtocol) {
         self.init(prng: prng, serializer: PlainSerializer())
     }
 
-    init(
+    convenience init(
         prng: PRNGProtocol,
         crypto: OpenVPNCryptoProtocol,
         authKey key: OpenVPN.StaticKey,
@@ -65,7 +65,7 @@ actor ControlChannel {
         self.init(prng: prng, serializer: try AuthSerializer(with: crypto, key: key, digest: digest))
     }
 
-    init(
+    convenience init(
         prng: PRNGProtocol,
         crypto: OpenVPNCryptoProtocol,
         cryptKey key: OpenVPN.StaticKey

Packages/PassepartoutOpenVPNOpenSSL/Sources/PassepartoutOpenVPNOpenSSL/Internal/DataChannel.swift

@@ -27,7 +27,7 @@ internal import CPassepartoutOpenVPNOpenSSL
 import Foundation
 import PassepartoutKit
 
-actor DataChannel {
+final class DataChannel {
     let key: UInt8
 
     private let dataPath: DataPath