For the performance of gethostbyname, Add dns cache hash table
urlmaps<url, ip>.

But, When I use fork to handle every http request, The hash table
can not be shared for all the new child process.
I need to choose some ipc method to fix this issue.
or change to use select/poll/epoll....

Signed-off-by: vulcan.lin <ssjmhyvi@gmail.com>