Add setSleeping callback

- Allows app to inform library when
  device sleeps/wakes.

- Sleep state overrides hasNetworkConnectivity
  state check, indicating no network when
  in sleep state.

- Logs sleep/wake event diagnostics.

MobileLibrary/iOS/PsiphonTunnel/PsiphonTunnel/PsiphonTunnel.h

@@ -356,6 +356,12 @@ Swift: @code func onInternetReachabilityChanged(_ currentReachability: Reachabil
  */
 - (void)stop;
 
+/*!
+ Indicate if the device is sleeping. This logs a diagnostic message and forces hasNetworkConnectivity to false when sleeping.
+ Swift: @code func setSleeping(_ isSleeping: Bool) @endcode
+ */
+- (void)setSleeping:(BOOL)isSleeping;
+
 /*!
  Returns the current tunnel connection state.
  @return  The current connection state.

MobileLibrary/iOS/PsiphonTunnel/PsiphonTunnel/PsiphonTunnel.m

@@ -53,6 +53,8 @@
     _Atomic NSInteger localSocksProxyPort;
     _Atomic NSInteger localHttpProxyPort;
 
+    _Atomic BOOL isSleeping;
+
     Reachability* reachability;
     _Atomic NetworkStatus currentNetworkStatus;
 
@@ -81,6 +83,7 @@
     atomic_init(&self->connectionState, PsiphonConnectionStateDisconnected);
     atomic_init(&self->localSocksProxyPort, 0);
     atomic_init(&self->localHttpProxyPort, 0);
+    atomic_init(&self->isSleeping, FALSE);
     self->reachability = [Reachability reachabilityForInternetConnection];
     atomic_init(&self->currentNetworkStatus, NotReachable);
     self->tunnelWholeDevice = FALSE;
@@ -178,6 +181,12 @@
     return [self start];
 }
 
+// See comment in header
+- (void)setSleeping:(BOOL)isSleeping {
+    [self logMessage: isSleeping ? @"Sleeping" : @"Waking"];
+    atomic_store(&self->isSleeping, isSleeping);
+}
+
 /*!
  Start the tunnel. If the tunnel is already started it will be stopped first.
  Assumes self.sessionID has been initialized -- i.e., assumes that
@@ -1080,6 +1089,11 @@
 }
 
 - (long)hasNetworkConnectivity {
+
+    if (atomic_load(&self->isSleeping)) {
+        return FALSE;
+    }
+
     BOOL hasConnectivity = [self->reachability currentReachabilityStatus] != NotReachable;
 
     if (!hasConnectivity) {