Better timeout recording and handling

Hello @nicks, @dpup, 

Please review the following commits I made in branch 'xiao-better-timeout-handling'.

882b5713c805a540a8134e63ad711a7f859dbd6c (2013-12-06 11:42:39 -0800)
Better timeout recording and handling
(1) Move response time recording to `_makeNodeResolverWithTimeout`, so
    we don't need a separate function to do that.

(2) Response time will be recorded even if a response gets timeout but
    finally goes through.

(3) Record timeout count for each individual operation.

Check list:


R=@nicks
R=@dpup