Previously it was checking TotalConns but that includes ConstructingConns.
Instead it should directly check IdleConns so the next Acquire takes one of
those and doesn't make a 3rd connection. The check against the context was
also wrong which prevented this from timing out after 2 minutes.
This also fixes a bug where NewConnsCount was not correctly counting
connections created by Acquire directly.
Fixes#1690
James Hartig
e665f74c99