feat: temporary session pool scaling?

[jxsl13]

now that we have a channel instead of a queue, we can now define a custom time based (and maybe event time + number of already active transient sessions) "session scaling" policy that allows to return a transient session after waiting for a while for sessions to be returned before trying to create a new transient connection + session.

[jxsl13]

something along the lines of this:

select {
    // sp = session pool
    case <-sp.ctx.Done():
        return nil, ErrClosed
    case <-ctx.Done():
        return nil, ctx.Err()
    case s, ok <- sp.sessions:
        if !ok{
            return nil, ErrClosed
        }
        return s, nil
    default:
        timeout, scale := sp.AutoScalePolicy(s.ActiveTransientConnections, s.ActiveTransientSessions)
        var waitChan chan time.Time = nil // blocking forever
        if scale {
            waitChan = time.After(timeout) // we might need to use an actual time.Timer here that can be properly closed when it is not needed anymore
        }

        select {
            case <-sp.ctx.Done():
                return nil, ErrClosed
            case <-ctx.Done():
                return nil, ctx.Err()
            case s, ok <- sp.sessions:
                if !ok {
                    return nil, ErrClosed
                }
                return s, nil
            case <-waitChan:
                return sp.GetTransientSession(ctx) // might need to use an unguarded variant of the method here
        }
    }
}