Initialize the schema in a transaction

It's probably not strictly necessary but it's sort of nice.

src/PaymentServer/Persistence.hs

@@ -337,6 +337,7 @@ sqlite path =
       dbConn <- Sqlite.open (unpack path)
       let exec = Sqlite.execute_ dbConn
       exec "PRAGMA foreign_keys = ON"
+      Sqlite.withExclusiveTransaction dbConn $ do
         exec "CREATE TABLE IF NOT EXISTS vouchers (id INTEGER PRIMARY KEY, name TEXT UNIQUE)"
         exec "CREATE TABLE IF NOT EXISTS redeemed (id INTEGER PRIMARY KEY, voucher_id INTEGER, counter INTEGER, fingerprint TEXT, FOREIGN KEY (voucher_id) REFERENCES vouchers(id))"
         return dbConn