make multiple export tasks work

The changes to the parameter file allow multiple export tasks to run.
They work around a bug with the export task that causes crashes when
maxduration < wfduration for any export task. The problem only occurs
when more than one export task is running and trying to write to the
same database.

It seems that as long as pktchannel2buffer_put doesn't have to flush a
buffer before it finishes writing out a file (maxduration >= wfduration)
for each export task, everything is happy. However if either (or both)
export task(s) has/have it's maxduration < wfduration, the crash occurs.