Feature request - show progress in "Verifying Blocks" and "Rescanning"

Hi devs,

I'm thinking about starting zcashd from the UI if it's not already running. I'd like to see some progress information during the startup phase when calling getclientinfo. Something like:

 {"code":-28,"message":"Verifying blocks...","progress":10}
 {"code":-28,"message":"Rescanning...","progress":20}

That way I can show a nice progress bar to the user

Thanks
zab