Add transaction versioning, tx cancellation (#79)

migrations/1503945570220-add-tx-version.js

@@ -0,0 +1,14 @@
+var db = require('./db')
+
+exports.up = function (next) {
+  const sql = [
+    'alter table cash_in_txs add column tx_version integer not null',
+    'alter table cash_out_txs add column tx_version integer not null'
+  ]
+
+  db.multi(sql, next)
+}
+
+exports.down = function (next) {
+  next()
+}