Make the pool size configurable about vinyl_v1 HOT 1 OPEN

Hi @dvarrazzo The thing is this is a sync backend, and DatabaseWrapper instances are created per-thread. So it means that I have 1 pool per thread. The pool is actually not needed at all because 1 thread only needs 1 connection at a time.

But I use the pool here as a logical entity. Because even if you need only one connection, that connection also has a lifetime after which it should be recreated, the health checks should be made from time to time, etc - the very same things you do to connections in the pool. So I need the pool just to reuse the logic of working with a single connection.

Now, about the size: I think, size=1 corresponds to the current logic of django's own postgresql backend. But you can probably have 2 connections when you know 1 connection is about to die so that to return the new connection immediately. But that is kind of optimisation.

from vinyl_v1.