Convert HcalRealisticZS to stream module

The HcalRealisticZS uses the HcalDbService which the static analyzer
has complained about because of that object's use of the HCAL
conditions containers. However, upon further investigation the HcalDbService
is thread safe for this use. In addition, an upcoming change to
the conditions containers will solve the remaining thread unsafe uses
of those containers.
With that in mind, it is safe to convert HcalRealisticZS to a stream module.