Rowcache child

@shrutip @alainjobart 
With this change, memcache is launched by vttablet on AllowQueries, and shut down on DisallowQueries. So, it doesn't have to be purged. This simplifies many things. In the config, you have to specify the command line parameters for memcache, and the rowcache parameters (like pool size) are inferred from those options.

rowcache_invalidator.py & update_stream.py are broken, but I couldn't figure out why. I think they were broken before these changes also.
I think my invalidator changes are superficial. @shrutip may want to look at what else should be deleted.