Prescale the muon shower lines

Test prescaling the muon shower lines, as they seem to take a lot of BW on data (900 MB/s).

A test on run 305589, shows:

nominal: 1.1G Sep  9 22:04 test.turbo.mdf
prescaled to 10%: 779M Sep  9 23:01 test.turbo.mdf

so a deducting of about 30% in the Turbo stream out of HLT2.