Fix Transport::flush implementation for TransportThread (#678)

Description for the `flush` function of the `Transport` trait states that
"If the queue was successfully drained, the return value should be `true`.",
but implementation of the trait on `tokio_thread::TransportThread`
do the oposite. The same error for `thread::TransportThread`.

Co-authored-by: Francesco Vigliaturo <francesco.vigliaturo@sentry.io>

sentry/src/transports/thread.rs

@@ -80,7 +80,7 @@ impl TransportThread {
     pub fn flush(&self, timeout: Duration) -> bool {
         let (sender, receiver) = sync_channel(1);
         let _ = self.sender.send(Task::Flush(sender));
-        receiver.recv_timeout(timeout).is_err()
+        receiver.recv_timeout(timeout).is_ok()
     }
 }
 

sentry/src/transports/tokio_thread.rs

@@ -96,7 +96,7 @@ impl TransportThread {
     pub fn flush(&self, timeout: Duration) -> bool {
         let (sender, receiver) = sync_channel(1);
         let _ = self.sender.send(Task::Flush(sender));
-        receiver.recv_timeout(timeout).is_err()
+        receiver.recv_timeout(timeout).is_ok()
     }
 }