bark: require txindex on bitcoind chain source

lucarequested to merge
luca/eng-318-properly-enforce-that-bark-and-barkd-require-txindex into master
  • check or remove this line after you've added a CHANGELOG entry or if your PR doesn't need one (they should go into CHANGELOG/unreleased/)

This MR makes bark fail-fast when using a bitcoind without txindex instead of erroing later on the first call that needs it.

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