Split dbfixtures into smaller packages

[fizyk]

Splitting dbfixtures into smaller packages each responsible for it's own db engine, would allow to ease the maintenance of such part, and at the same time woul dleave dbfixtures as a metapackage

[fizyk]

in example old version of pytest-elasticsearch could support older elasticsearch engines, why newer could only support newer ones.

[fizyk]

• elasticsearch -> ClearcodeHQ/pytest-elasticsearch
• mongodb -> ClearcodeHQ/pytest-mongo
• postgresql -> ClearcodeHQ/pytest-postgresql
• dynamodb -> ClearcodeHQ/pytest-dynamodb
• mysql -> ClearcodeHQ/pytest-mysql
• rabbitmq -> ClearcodeHQ/pytest-rabbitmq
• redis -> ClearcodeHQ/pytest-redis