Start implementation of parallel burnin

This patch introduces a simple framework for executing jobs in parallel
in burnin (the ExecJobSet function) and the "--parallel" command line
flag.

The patch also changes the instance creation to run in parallel when the
above flag is given. Error handling/instance removal is currently flacky
with this options if there are errors in the instance creation.

We also modify burnin to reuse a single client.

Reviewed-by: imsnah