Added idempotent insert, insertMany, delete, deleteMany methods

Deprecated `update`, `updateMany`, `upsert`, `upsertMany` as there is no way to ensure idempotence of these methods and these methods should be implemented via `withTransaction` in user code. Intention is to remove them completely at a later stage.
Merge pull request #23 from ovangle/idempotence

Added idempotent insert, insertMany, delete, deleteMany methods