expose database pool's maximum size, fixes #494

[brontolosone]

Exposes database pool's maximum size.

Closes #494

What has been done to verify that this works as intended?

• Verified that a config's default.database.maximumPoolsize actually has an effect. It does.
• CI

Why is this the best possible solution? Were any other approaches considered?

N/A

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

N/A

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No. We didn't do it for #1404 which is very similar. But here I'm floating the idea of creating a scaling guide.

Before submitting this PR, please make sure you have:

• branched off and targeted the next branch OR only changed documentation/infrastructure (master is stable and used in production)
• verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced