no guarantee of order of results from execParallel

[SharonU]

execParallel is based on caolan/async's each function, on which is noted:

"Note, that since this function applies the iterator to each item in parallel there is no guarantee that the iterator functions will complete in order."

[mgutz]

There is also execSeries. I added execParallel for symmetry. TBH, I rarely used it. I used it in testing to delete data from a set of tables without concern for order.