Fix Pool's ongoing count

[HexyWitch]

I encountered an issue where after some time the server would start timing out trying to connect to the mysql server. I'm fairly sure I've narrowed it down to the ongoing count in Pool not always being decremented when connections are dropped or disconnected. As soon as it reaches the PoolConstraints max, and the idle pool has been exhausted, the Pool stops creating any new connections and locks up forever.

I don't have a minimal reproduction case, so instead have this long description of why I think this change is correct:

Ongoing count is incremented when:

1. A new connection has been created and initiated (sets the inner pool)
2. A connection is popped from the idle connections pool

With my changes the count is now decremented when:

1. A connection is dropped or disconnected (if it has an inner pool)
2. A connection is pushed to the idle connections pool

I believe this behavior is correct as long as:

• Conn::pool is always set when a connection is taken from the pool
• Conn::pool is never reset while the connection is ongoing
• Conn::pool is always immediately reset when the connection is disconnected

It seems stable in local testing under high stress, and in production.

[blackbeam]

Thanks! I'll look into it.