Allow setting a minimum number of ready to use resources to be created

[ludat]

So I've got this use case where I need to connect to a db and the first connection is somewhat expensive, so I'd like to keep a connection always ready to avoid having the user waiting for the app to connect for the first time.

My idea is just like there is the maximum number of resources, there's also a minimum which never gets freed no matter what.

That could also help with #33 because if the user is eager to create the connection that would show an error immediately instead of the first time the pool needs to create a resource.

And maybe it's time to reify the settings for createPool since it starting to have too many parameters (which are similar in types but different in semantics)

Also thanks for writing such a useful library :D

[domenkozar]

For context also read https://qnikst.brick.do/2020-12-28-resource-pool