Add Multipath TCP (MPTCP) support

MPTCP is a TCP extension allowing to improve network reliabilty by using mutiple network interface for the same TCP connection. Check https://www.mptcp.dev and https://developer.apple.com/documentation/foundation/urlsessionconfiguration/improving_network_reliability_using_multipath_tcp for details.
Changes to this repository consists in setting the multipathServiceType of URLSessionConfiguration to Handover, allowing a seamless transition of interface in case of deterioration
of the connection.

SignalServiceKit/Network/ContentProxy.swift

@@ -25,6 +25,7 @@ public class ContentProxy: NSObject {
             "HTTPSProxy": proxyHost,
             "HTTPSPort": proxyPort
         ]
+        configuration.multipathServiceType = .handover
         return configuration
     }
 

SignalServiceKit/Network/OWSUrlSession.swift

@@ -81,13 +81,16 @@ public class OWSURLSession: NSObject, OWSURLSessionProtocol {
     }
 
     public static var defaultConfigurationWithCaching: URLSessionConfiguration {
-        .ephemeral
+        let configuration = URLSessionConfiguration.ephemeral
+        configuration.multipathServiceType = .handover
+        return configuration
     }
 
     public static var defaultConfigurationWithoutCaching: URLSessionConfiguration {
         let configuration = URLSessionConfiguration.ephemeral
         configuration.urlCache = nil
         configuration.requestCachePolicy = .reloadIgnoringLocalCacheData
+        configuration.multipathServiceType = .handover
         return configuration
     }
 

SignalServiceKit/Subscriptions/Stripe.swift

@@ -270,11 +270,17 @@ fileprivate extension Stripe {
         : "pk_test_sngOd8FnXNkpce9nPXawKrJD00kIDngZkD"
 
     static let authorizationHeader = "Basic \(Data("\(publishableKey):".utf8).base64EncodedString())"
+
+    static let configuration: URLSessionConfiguration{
+        let configuration = URLSessionConfiguration.ephemeral
+        configuration.multipathServiceType = .handover
+        return configuration
+    }
 
     static let urlSession = OWSURLSession(
         baseUrl: URL(string: "https://api.stripe.com/v1/")!,
         securityPolicy: OWSURLSession.defaultSecurityPolicy,
-        configuration: URLSessionConfiguration.ephemeral
+        configuration: configuration
     )
 
     struct API {

SignalUI/LinkPreview/LinkPreviewFetcher.swift

@@ -102,6 +102,7 @@ public class LinkPreviewFetcherImpl: LinkPreviewFetcher {
         let sessionConfig = URLSessionConfiguration.ephemeral
         sessionConfig.urlCache = nil
         sessionConfig.requestCachePolicy = .reloadIgnoringLocalCacheData
+        sessionConfig.multipathServiceType = .handover
 
         // Twitter doesn't return OpenGraph tags to Signal
         // `curl -A Signal "https://twitter.com/signalapp/status/1280166087577997312?s=20"`

SignalUI/Payments/MobileCoinAPI+Configuration.swift

@@ -723,6 +723,7 @@ final class MobileCoinHttpRequester: NSObject, HttpRequester {
         let config = URLSessionConfiguration.default
         config.timeoutIntervalForRequest = 30
         config.timeoutIntervalForResource = 30
+        config.multipathServiceType = .handover
         return config
     }()