Fix race in QueuedWorkerPool shutdown, which was resulting in a test seg-fault 5% of the time.  We need to avoid referencing locals in NextFunction() after signalling that seqeunce shutdown is OK.           Keep track of whether shutdown is currently occurring and thus we should  not initiate handling of any new requests. 
Fix race in QueuedWorkerPool shutdown, which was resulting in a test
seg-fault 5% of the time.  We need to avoid referencing locals in
NextFunction() after signalling that seqeunce shutdown is OK.
         
Keep track of whether shutdown is currently occurring and thus we should 
not initiate handling of any new requests. 



ajayanandgit/mod_pagespeed,patricmutwiri/mod_pagespeed,jalonsoa/mod_pagespeed,pagespeed/mod_pagespeed,jalonsoa/mod_pagespeed,wanrui/mod_pagespeed,webscale-networks/mod_pagespeed,pagespeed/mod_pagespeed,jalonsoa/mod_pagespeed,VersoBit/mod_pagespeed,webscale-networks/mod_pagespeed,pagespeed/mod_pagespeed,patricmutwiri/mod_pagespeed,ajayanandgit/mod_pagespeed,VersoBit/mod_pagespeed,patricmutwiri/mod_pagespeed,ajayanandgit/mod_pagespeed,hashashin/src,wanrui/mod_pagespeed,jalonsoa/mod_pagespeed,wanrui/mod_pagespeed,patricmutwiri/mod_pagespeed,wanrui/mod_pagespeed,hashashin/src,hashashin/src,pagespeed/mod_pagespeed,webhost/mod_pagespeed,patricmutwiri/mod_pagespeed,hashashin/src,jalonsoa/mod_pagespeed,wanrui/mod_pagespeed,pagespeed/mod_pagespeed,ajayanandgit/mod_pagespeed,webhost/mod_pagespeed,VersoBit/mod_pagespeed,webscale-networks/mod_pagespeed,wanrui/mod_pagespeed,webhost/mod_pagespeed,wanrui/mod_pagespeed,VersoBit/mod_pagespeed,webhost/mod_pagespeed,hashashin/src,patricmutwiri/mod_pagespeed,webhost/mod_pagespeed,webhost/mod_pagespeed,hashashin/src,patricmutwiri/mod_pagespeed,patricmutwiri/mod_pagespeed,ajayanandgit/mod_pagespeed,webscale-networks/mod_pagespeed,pagespeed/mod_pagespeed,webscale-networks/mod_pagespeed,jalonsoa/mod_pagespeed,VersoBit/mod_pagespeed,pagespeed/mod_pagespeed,jalonsoa/mod_pagespeed,webscale-networks/mod_pagespeed,webscale-networks/mod_pagespeed,wanrui/mod_pagespeed,VersoBit/mod_pagespeed,ajayanandgit/mod_pagespeed,VersoBit/mod_pagespeed,ajayanandgit/mod_pagespeed,hashashin/src,webscale-networks/mod_pagespeed,hashashin/src,webhost/mod_pagespeed,webhost/mod_pagespeed,ajayanandgit/mod_pagespeed,jalonsoa/mod_pagespeed