Teach sky_server how to use more threads and not drop requests

This is lame, but seems to fix the timeout problem
http://stackoverflow.com/questions/2685089/cherrypy-and-concurrency

Review URL: https://codereview.chromium.org/700213005

engine/src/flutter/tools/sky_server

@@ -43,6 +43,10 @@ def main():
 
     config = {
         'global': {
+            # Cherrypy appears to just drop requests if it doesn't have
+            # a thread to service them, so set our thread_pool high enough
+            # so that a z620 can run all the tests w/o timeouts.
+            'server.thread_pool': 30,
             'server.socket_port': args.port,
             'tools.staticdir.content_types' : {
                 'sky': 'text/sky',