Demarcating checkin and checkout of a physical connection from a pool

[mp911de]

See also https://groups.google.com/g/r2dbc/c/zZ09hFMR9N8:

The JDBC Spec defines beginRequest and endRequest methods that allows connection pools to notify physical connections on checkin/checkout.
We believe that If JDBC needs such API, then there's a good chance R2DBC needs a reactive version of that same API.

[mp911de]

I create a pull request #235 to explore lifecycle callbacks as per this ticket.