Redmine #882: Change type to indicate non-participation in reductions
Non-participation in reductions is now indicated by the 'nop'
reducer type, and empty contributes are now implemented internally
using 'nop'. Previously this was done with the 'random' reducer type.
Note that the reduction framework has optimizations to ignore/skip
most of these messages, so previous use of nop might be faster now,
while previous use of random might be slower, but random will now do
as expected (see below).
The 'random' reducer function has been changed to its intended
behavior (to select a random message); previously it always chose
the first message.