Add Connection Pooling

[oubiwann]

In order to take kanin to the next level of usability and robustness, it needs to support some form of connection pooling.

This may or may not be an actual implementation (it may be just documentation), but very likely a minimal default will be provided.

Features and tasks for this epic:

• Evaluate connection pooling options - Evaluate connection pooling options #12
• Support for creating pools on demand - Support for creating pools on demand #18

Part of release v0.8.0