MB-11917: Bump reductions before the NIF call

The mapreduce NIF probably screws up the scheduler. Hence aggressively
increase the reduction count. This should prevent the schedulers from
blocking.

The idea comes from the #erlang IRC channel, the implementation is
similar to the one from Basho's eleveldb.

Change-Id: I330b8466a1b559837b2c787c13d2130589f7193b
Reviewed-on: http://review.couchbase.org/40591
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>