Suggest threadpool size to Twisted.

Take a guess at how many threads we need.  We need one for each consumer
thread, plus one for each producer, plus an extra "one to grow on".

This should let us scale the number of FMN workers past 4.  As things
stand now, if we ask for more than 4 FMN workers, then there isn't an
extra thread around to drive the monitoring polling producer, and NRPE
starts timing out due to lack of data.