NcWMS: async date loading

on slower machines dates loading can jam the browser as it was done
inline. the solution is to interleave it and load every year in an async
manner. this lets the browser regain control and the experience is more
responsive
NcWMS: interleave with setTimeout when date loaded

if a date is already loaded, use setTimeout to ensure the first thing we
do is release control back to the ui, resulting in a happier user