shutdown-sequence: fix a segfault during a shutdown under load
shutdown-sequence: fix a segfault during a shutdown under load

Shut down the driver factory before the proxy fetch factory.
shutting down the driver factory will make sure that all worker
threads are stopped, This prevents in-flight callbacks to
destroyed proxy fetch factories, so we should be able to safely
delete them

This fixes #109.

digideskio/ngx_pagespeed,0x0mar/ngx_pagespeed,pagespeed/ngx_pagespeed,pagespeed/ngx_pagespeed,0x0mar/ngx_pagespeed,wanrui/ngx_pagespeed,wanrui/ngx_pagespeed,digideskio/ngx_pagespeed