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.
Merge pull request #18 from mokshaproject/feature/threadpool-size

Suggest threadpool size to Twisted.
Make the monitoring socket clean itself up nicely.
Configurable permissions on monitoring socket.

We have the problem in the Fedora Infrastructure production environment
where moksha needs to be able to write to the socket, but nrpe needs to
also be able to write to the socket (to set its subscription).
Currently, we have the socket owned by fedmsg, but the gid is set to
nrpe.  The default permissions, however, are for read-only access on the
group bits.

This change will allow us to tell moksha to set the permissions on the
socket to 0770 at startup, and thus will cut down on UNKNOWN responses
from nrpe.