Add switch to allow aliases to be processed in parallel

[schlessera]

It would be useful to allow aliases to be processed in parallel, so that the machine controlling the process can open multiple connections at the same time. Under the right circumstances, this would drastically improve the speed of execution across multiple machines.

From @lukasleitsch via https://twitter.com/lukasleitsch/status/955749917544329216

[danielbachhuber]

That'd be pretty nifty. It'd even improve performance running across multiple installs on the same machine.