Pool-less mode

In scenarios where multiple pools access the same data source instance you may end up in a situation where the combined number of max-pool-size is greater than the maximum number of connections in the data source.

Introduce a pool-less mode, where the underlying connection is closed upon return to the pool if the pool size is greater than min-pool-size.

This will allow you to configure Agroal to
.mode("pool-less").minSize(0).maxSize(1)
that only use 1 connection that is returned upon close. This is especially useful for service-mesh scenarios where each service is short-lived, and process one transaction.