Add an option to enable concurrent reads and responses in HTTP/1

By default Go HTTP server consumes any unread request portion before
writing the response for HTTP/1. This prevents handlers from reading
request and writing response concurrently.

This is set to false by default since it might be an unexpected behavior
and cause deadlock for some handlers. See
golang/go#15527 (comment)

jobs/gorouter/spec

@@ -161,6 +161,9 @@ properties:
   router.enable_http2:
     description: Enables support for HTTP/2 ingress traffic to the Gorouter. Also enables the option to use the HTTP/2 protocol for traffic to specified backends.
     default: true
+  router.enable_http1_concurrent_read_write:
+    description: Enables concurrent request reads and response writes for HTTP/1 requests
+    default: false
   router.min_tls_version:
     description: Minimum accepted version of TLS protocol. All versions above this, up to the max_tls_version, will also be accepted. Valid values are TLSv1.0, TLSv1.1, TLSv1.2, and TLSv1.3.
     default: TLSv1.2

jobs/gorouter/templates/gorouter.yml.erb

@@ -438,6 +438,7 @@ end
 
 params['disable_http'] = p('router.disable_http')
 params['enable_http2'] = p('router.enable_http2')
+params['enable_http1_concurrent_read_write'] = p('router.enable_http1_concurrent_read_write')
 
 if p('router.ca_certs')
   params['ca_certs'] = get_valid_ca_certs