fixup! async_sender: factor out thread pool to a seprate class

spec/thread_pool_spec.rb

@@ -40,7 +40,9 @@
       end
 
       it "returns false" do
-        expect(subject << 1).to eq(false)
+        retval = subject << 1
+        subject.close
+        expect(retval).to eq(false)
       end
 
       it "discards tasks" do
@@ -94,6 +96,8 @@
     end
 
     context "when there's some work to do" do
+      let(:workers) { 0 }
+
       it "logs how many tasks are left to process" do
         expect(Airbrake::Loggable.instance).to receive(:debug).with(
           /waiting to process \d+ task\(s\)/