QC: add downsampling back to FastQC
QC: add downsampling back to FastQC

After more testing on larger files and slower filesystems,
downsampling does provide a big speed improvement.

lbeltrame/bcbio-nextgen,chapmanb/bcbio-nextgen,vladsaveliev/bcbio-nextgen,chapmanb/bcbio-nextgen,biocyberman/bcbio-nextgen,chapmanb/bcbio-nextgen,a113n/bcbio-nextgen,lbeltrame/bcbio-nextgen,biocyberman/bcbio-nextgen,vladsaveliev/bcbio-nextgen,a113n/bcbio-nextgen,vladsaveliev/bcbio-nextgen,brainstorm/bcbio-nextgen,biocyberman/bcbio-nextgen,lbeltrame/bcbio-nextgen,brainstorm/bcbio-nextgen,a113n/bcbio-nextgen,brainstorm/bcbio-nextgen