do the read/write operations from the db should be in a transaction

`redeemVoucher' reads stuff from the db, makes decisions and then
writes to the db. However if there is another client doing a
simultaneous `redeemVoucher' for the same `voucher', then both of them
would redeem the voucher, which is incorrect. The operations should
have exclusive access to the db and this is achieved with
`withExclusiveTransaction`.

src/PaymentServer/Persistence.hs

@@ -101,7 +101,7 @@ instance VoucherDatabase VoucherDatabaseState where
     let insertFn = (modifyIORef redeemed .) . Map.insert
     redeemVoucherHelper (unpaid, existingFingerprint) voucher fingerprint insertFn
 
-  redeemVoucher SQLiteDB { conn = conn } voucher fingerprint = do
+  redeemVoucher SQLiteDB { conn = conn } voucher fingerprint = Sqlite.withExclusiveTransaction conn $ do
     unpaid <- isVoucherUnpaid conn voucher
     existingFingerprint <- getVoucherFingerprint conn voucher
     let insertFn = insertVoucherAndFingerprint conn