Add connection_holder_class to Pool for custom connection handling

[KorsaR-ZN]

This PR introduces the connection_holder_class parameter to the Pool class, enabling developers to plug in custom logic for managing individual connections — including acquisition, release, and lifecycle control. This added flexibility is particularly useful for advanced or non-standard use cases where the default connection handling may be insufficient.

The change also lays the groundwork for addressing issue #989 by making it possible to implement custom connection management strategies tailored to specific requirements.

[KorsaR-ZN]

If you consider the behavior described in #989 to be a bug in connection management, I also have a modified version of PoolConnectionHolder that addresses it.

This version ensures that connections are not closed below min_size, even in cases where the database drops the connection. I'm happy to submit it as a separate PR if there's interest.

[elprans]

If you consider the behavior described in #989 to be a bug in connection management, I also have a modified version of PoolConnectionHolder that addresses it.

I think #989 is arguably a bug.

[KorsaR-ZN]

If you consider the behavior described in #989 to be a bug in connection management, I also have a modified version of PoolConnectionHolder that addresses it.

I think #989 is arguably a bug.

Great, I also consider the current behavior to be a bug. In that case, I suggest we fix it as part of this PR — I'll update it a bit later to include the improved PoolConnectionHolder

[elprans]

Great, I also consider the current behavior to be a bug. In that case, I suggest we fix it as part of this PR — I'll update it a bit later to include the improved PoolConnectionHolder

I would prefer if we separated the fix into a standalone PR.

PoolConnectionHolder is really a fairly low-level implementation detail and I'm a bit wary of turning it into a public API. People already tend to misuse private methods with custom connection/pool classes and things break when we change things (as simple as adding a new argument to a private method). It might be more practical to allow more configurability to Pool via a policy callback or something like that rather than a full subclass.

[KorsaR-ZN]

@elprans, I understand your concerns about making PoolConnectionHolder part of the public API, as it is quite low-level. However, it seems that PoolConnectionHolder is essentially the connection lifecycle management policy within the pool. We might just need to simplify it a bit and make it more suitable for public use, which is what I originally planned to do.

Regarding the idea of adding a separate policy, callback, or something similar, I’ll consider it, but it seems that this would only add extra complexity due to the additional layer of abstraction.