Fix the C++ client example to start listening for certificateProvider events before the NaCl module is loaded

[emaxx-google]

Currently, the provided C++ client example implementation starts listening for chrome.certificateProvider events only after the NaCl module has been loaded.

As the loading of the NaCl module may take a considerable time (several seconds or even dozens of seconds), this means that user may not be able to start opening HTTPS-with-client-auth websites for quite some time (all such attempts would finish in HTTPS error in Chrome).

So this should be improved, by setting up the required listeners straight at the app startup code. The requests, that come before the NaCl module has finished loading, should be stored somewhere and delayed.

/cc @dskaram