12084: Fix dispatcher getting bogged down on "too many open files".

12084: Fix dispatcher getting bogged down on "too many open files".

Previously, handling the "too many open files" error from popen3
consisted of sleeping 1 second and trying the next job in the
queue. When lots of jobs were queued, this meant getting stuck in
start_jobs() for a long time, futilely trying to start different jobs,
which all failed for the same reason.

Worse, being stuck here meant none of the working jobs could finish,
which meant no file descriptors could be freed.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tclegg@veritasgenetics.com>