CURATOR-151: SharedValue/SharedCount API update

Ah, gotcha.  So this isn't new behavior -- this is just a standard compare-and-set loop that replaces the synchronized keyword.  It's not round tripping to the server multiple times.  There's still only one attempt to set the server value, and if it fails the calling methods return false.  This is just atomically setting currentValue.  We could replace the loop with a synchronized block, but performance would be worse.