Add an option to pause rampup

**oliver lloyd** ([Bug 53317](https://bz.apache.org/bugzilla//show_bug.cgi?id=53317&redirect=false)):
Presently, once a test has begun, then the rampup logic will continue to add threads at the defined rate until the rampup period has expired.

This is a RFE for:

1. An option to pause the rampup such that any threads that have already been started continue to execute but no new threads are added
2. A related option to resume the rampup and continue to add threads at the previous rate.

Votes in Bugzilla: 1
OS: All