Implement pux_fetch_mux and pux_store_mux to support persistent mux.

[c9s]

to reach the goal of #18, we should first implement the basic functions to store/fetch mux.

currently we've done the pux_store_mux function, now we need to implement pux_fetch_mux.

the API is something like APC, but we store the zval without serialization/unserialization:

if (  ($mux = pux_fetch_mux('app_pmux')) == null ) {
   $mux = new Mux;
   pux_store_mux('app_pmux', $mux);
}

[c9s]

Done!