How should round duration be set?

[Zerim]

Need to balance trade-offs:

• Shorter round duration leads to better market signals (because aggregate transaction value per subgraph being computed more often)
• Longer round duration leads to less gas costs to run the protocol (because paying out inflation rewards requires transactions on-chain).

Note, we've deferred this question to governance in the spec, but we should still analyze how this parameter might be set.