Race condition with fund update

[yuriytkach]

If two http requests come to track donation simultaneously it can start 2 lambda functions in parallel. Then we'll have a race condition in updating fund total - both lambdas will read the current fund total, then add a sum and then write. This can lead to one write overriding the other. We need to resolve that with DynamoDB conditional writes.

We could have used sqs to publish event there and then process them one by one, but that will require to have SQS. If we're still OK and within the processing time limit from Monobank to perform conditional write, then that could be easier to implement.