Added Optimistic Concurrency handling

[oskardudycz]

Fixes #52

Added implementation of conventional-based _version and expectedVersion handling.

As it's a bigint type, I had to also add JSONSerializer with custom bigint handling.
WARNING: Now, all numeric strings will be deserialised as bigint type.

TODO:

• Add custom documents code to enable custom field (de)serialisation,
• Upsert handling for update or explicit upsert function