Unblock connection receive when context is done

When context is canceled we don't read anymore from `t.incoming` channel
because the select clause exists on `ctx.Done`. This change adds the
same behavior to the point where `t.incoming` channel is written.
Without this the call to `conn.Receive` could block forever given a
race condition between a context cancellation and a new incoming
message.

Furthermore, is always a good practice to listen to `context.Done` on
blocking points where a context variable is available.

Signed-off-by: Iñigo Garcia Olaizola <11333576+igolaizola@users.noreply.github.com>

protocol/pubsub/v2/protocol.go

@@ -201,7 +201,10 @@ func (t *Protocol) startSubscriber(ctx context.Context, sub subscriptionWithTopi
 	}
 	// Ok, ready to start pulling.
 	return conn.Receive(ctx, func(ctx context.Context, m *pubsub.Message) {
-		t.incoming <- *m
+		select {
+		case t.incoming <- *m:
+		case <-ctx.Done():
+		}
 	})
 }