reflow1.19.0

Highlights from this release:

• various under the hood improvements to reduce Reflow’s memory usage
  • if you’ve upgraded an adhoc in the past due to Reflow’s memory requirements, now would be a good time to re-evaluate
  • in particular, we strongly encourage users to update dir summation code to take advantage of these improvements

Change this:  dirs.Make(map([(fooPath, fooFile)] + list(otherDir)))
to this:      dirs.Make(map([(fooPath, fooFile)])) + otherDir

• reflow can now launch instances in multiple availability zones
• reflow info output now includes the error message for failed execs, see reflow info -help for more info
• various minor bug fixes and internal refactoring