Add smarter autotuning for max postgres connections based on enabled roles / Synapse worker counts.

As for why i even came across this idea. A friend helping a friend of theirs who run a public synapse using the playbook ran out of postgres connections seemingly on 500 and i think i saw one of each as the pattern the workers displayed.

I should note that the exact details of this other setup that ran into trouble prompting this feature request are unknown to me so i wont be making any report on that.