Enhance Constant throughput timer

[asfimport]

Iulian Cutui (Bug 34739):
If you have more than one thread group with a constant througput timer in each
of them set in non legacy mode, then the actual throughput of each thread group
is false computed. The computations are made using
JMeterContextService.getNumberOfThreads(), but this method returns the total
number of threads from all thread groups. The computations should be made using
the number of threads from the current thread group.

OS: All

[asfimport]

Sebb (migrated from Bugzilla):
There are (at least) 3 possible ways of calculating the throughput:

• separately for each Thread (legacy mode = true)
• over all active threads (legacy mode = false)
• over all active threads in the thread group (not yet implemented - but I'm
  working on it..)

Seems to me that they are all equally valid.

Changing to an enhancement.

[asfimport]

Sebb (migrated from Bugzilla):
Added to CVS HEAD, along with fix for time calculation per thread