snpEff: auto-scale memory for minimal runs
snpEff: auto-scale memory for minimal runs

The new snpEff default of 3g does not provide enough memory for loading
the human database on a single core. Default to 2 times the per-core
memory for small runs to avoid this. Fixes #1730

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