[AltServer] Moves core ConnectionManager logic to AltKit

Refactors ConnectionManager to use arbitrary RequestHandlers and ConnectionHandlers. This allows the core AltServer request logic to be shared across different targets with different connection types.

AltKit/Connections/NetworkConnection.swift

@@ -0,0 +1,50 @@
+//
+//  NetworkConnection.swift
+//  AltKit
+//
+//  Created by Riley Testut on 6/1/20.
+//  Copyright © 2020 Riley Testut. All rights reserved.
+//
+
+import Foundation
+import Network
+
+public class NetworkConnection: NSObject, Connection
+{
+    public let nwConnection: NWConnection
+    
+    public init(_ nwConnection: NWConnection)
+    {
+        self.nwConnection = nwConnection
+    }
+    
+    public func __send(_ data: Data, completionHandler: @escaping (Bool, Error?) -> Void)
+    {
+        self.nwConnection.send(content: data, completion: .contentProcessed { (error) in
+            completionHandler(error == nil, error)
+        })
+    }
+    
+    public func __receiveData(expectedSize: Int, completionHandler: @escaping (Data?, Error?) -> Void)
+    {
+        self.nwConnection.receive(minimumIncompleteLength: expectedSize, maximumLength: expectedSize) { (data, context, isComplete, error) in
+            completionHandler(data, error)
+        }
+    }
+    
+    public func disconnect()
+    {
+        switch self.nwConnection.state
+        {
+        case .cancelled, .failed: break
+        default: self.nwConnection.cancel()
+        }
+    }
+}
+
+extension NetworkConnection
+{
+    override public var description: String {
+        return "\(self.nwConnection.endpoint) (Network)"
+    }
+}