Updated test helper

Includes a fix for limiting the number of retries when dropping test db.

Versionless Key-value store

• Adds a versionless, simple key value store to pg-doc

v2.0.0: Possibility to restore a removed document (#8)

* Possibility to restore a removed document

* (Soft) removing a document creates a empty version as the latest
  version (possible to list using force).
* Added possibility to specify correlation id when removing (added to
  the empty version specified above).
* Added possibility to restore an entity to a specific version. This
  will create a copy of the version and put it as the latest version.
  This will also remove the soft remove of the entity.
* Changed the behaviour of the remove function, the cb only takes an
  err, not a res. The error will be raised if no such entity exists.
* The changed behaviour is a breaking API change and thus the major
  version has been bumped.
* Removed check that entity is not already removed when removing