Fix migration to drop all tables at once
Fix migration to drop all tables at once

Foreign key constraints between the tables were preventing the migration
from chain/chainprv#654 from running successfully.

Closes chain/chainprv#659.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@jeffomatic

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain