More connections allowed in pool (db engine args)

[Theodlz]

As suggested by @profjsb, we never use more than 1/3 of the max nb of connections to the database. Here, we try to allow more connections at once to prevent the frontend apps from filling up the pool and timing out too quickly.

[Theodlz]

@profjsb are these the kind of numbers you had in mind? The defaults are 15 and 10, here I used 30 and 20.

[mcoughlin]

@Theodlz do we want to merge this?

[Theodlz]

@mcoughlin we can close it for now. It didn't create any issues recently, after improving the frontend to remove redundant calls.