HPCC-8248 - Allow frunssh to use more threads by default

This in particular effected backupnode, where the child processes
can take a long time. With the default frunssh limit of 10, it
meant that it would only start 10 (out of e.g. 400) slave
backupnode processes at a time, extending the overall time
significantly.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>