Optimistic Locking

## Background

Optimistic locking (or optimistic concurrency control) is a technique that allows concurrent edits on a single record (more: https://hexdocs.pm/ecto/0.9.0/Ecto.Model.OptimisticLock.html)

## Implementation

- [ ] Detect Optimistic Locking enabled when a struct contains a `LockVersion int` field.
- [ ] Use `LockVersion` field whenever Update or Delete is performed to prevent operation on stale version of record. Maybe we can simply add additional where filter LockVersion=? on update or delete query.
- [ ] Differentiate error when updating/deleting stale record for non existent record.