Use background task when sending analytics events.

// FREEBIE

SignalServiceKit/src/Util/OWSAnalytics.m

@@ -124,6 +124,7 @@ const int kOWSAnalytics_DiscardFrequency = 0;
 
 - (void)tryToSyncEvents
 {
+    dispatch_async(self.serialQueue, ^{
         // Don't try to sync if:
         //
         // * There's no network available.
@@ -132,7 +133,6 @@ const int kOWSAnalytics_DiscardFrequency = 0;
             return;
         }
 
-    dispatch_async(self.serialQueue, ^{
         __block NSString *firstEventKey = nil;
         __block NSDictionary *firstEventDictionary = nil;
         [self.dbConnection readWithBlock:^(YapDatabaseReadTransaction *transaction) {
@@ -157,6 +157,12 @@ const int kOWSAnalytics_DiscardFrequency = 0;
 
         DDLogDebug(@"%@ trying to deliver event: %@", self.tag, firstEventKey);
         self.hasRequestInFlight = YES;
+
+        __block UIBackgroundTaskIdentifier task;
+        task = [UIApplication.sharedApplication beginBackgroundTaskWithExpirationHandler:^{
+            [UIApplication.sharedApplication endBackgroundTask:task];
+        }];
+
         // Until we integrate with an analytics platform, behave as though all event delivery succeeds.
         dispatch_async(dispatch_get_main_queue(), ^{
             self.hasRequestInFlight = NO;
@@ -169,6 +175,8 @@ const int kOWSAnalytics_DiscardFrequency = 0;
                 }];
             }
 
+            [UIApplication.sharedApplication endBackgroundTask:task];
+
             // Wait a second between network requests / retries.
             dispatch_after(
                 dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.f * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{