Don't always automatically switch to an epoch when finding it

[Powersource]

Making this logic more refined #78 (comment)

Idk if this is a concrete todo yet or if this will maybe resolve itself when we fix other issues, as well as @mixmix working on the "preferred epoch" logic

situations we've found so far:

• ssb-tribes2/index.js

  Line 462 in d3073f2

  // TODO: naively guessing that this is the latest key for now

• ssb-tribes2/index.js

  Lines 444 to 445 in 4605738

  	// TODO: which writeKey should be picked??
  	// this will essentially pick a random write key