Reset cohort_variant when updating allele frequencies

Clear cohort_variant table and restart cohort_variant_id_seq sequence
when updating allele frequencies to prevent the 'integer out of range'
exception from happening during nightly recalculations.  Fixes #187.

Signed-off-by: Ryan O'Hara <oharar@email.chop.edu>
Merge pull request #223 from ryanjohara/issue-187

Reset cohort_variant when updating allele frequencies