[feature request] semi-permanent resource

Thanks for your great work, resource_cache is a very useful library.

Currently, there is a feature that I'm looking forward to.

For example, when the resources are connections to a database, I'd like to keep the connections permanent if they are not idle, instead of closing them after max_resource_reuse is reduced to zero, so there won't be too much connecting disconnecting(which is expensive).

So, is there any chance to redefine the
`max_resource_reuse` field in `Resource_cache.Config.t`,
as
`type max_resource_reuse= Unlimited | Reuse of int`
?
So we can alter the behavior of resource_cache more handily.

Start by reading Resource_cache.Config.t and the current max_resource_reuse handling. Trace how reuse limits affect idle and active resources; done means supporting Unlimited alongside Reuse of int while preserving existing finite-reuse behavior and preventing non-idle resources from being closed solely because the limit reaches zero.

Written by the indexing model from the issue text.