Fix runtime_client blocking main thread

runtime_client.next is calling into the C extension which blocks the
main thread. Moving it to a separate thread enables the main thread
to process signal, see this issue for more details: #105

Author: tgsong

Diff for: awslambdaric/lambda_runtime_client.py

@@ -5,6 +5,7 @@
 import http
 import http.client
 import sys
+from concurrent.futures import ThreadPoolExecutor
 from awslambdaric import __version__
 
 
@@ -65,7 +66,9 @@ def post_init_error(self, error_response_data):
             raise LambdaRuntimeClientError(endpoint, response.code, response_body)
 
     def wait_next_invocation(self):
-        response_body, headers = runtime_client.next()
+        with ThreadPoolExecutor() as e:
+            fut = e.submit(runtime_client.next)
+        response_body, headers = fut.result()
         return InvocationRequest(
             invoke_id=headers.get("Lambda-Runtime-Aws-Request-Id"),
             x_amzn_trace_id=headers.get("Lambda-Runtime-Trace-Id"),