Cardano Updates

## Description
withdrawal information for gov action is now always an array instead of
null or array;

updated tx_info endpoint's _governance output building to use json array
syntax to match earlier change to proposal_list endpoint

## Which issue it fixes?
<!--- Link to issue: Closes #issue-number -->

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Co-authored-by: hodlonaut <[email protected]>

Added logic for upper bound on epoch shown in drep voting power histo…y
to be the epoch of de-registration, if that was the last action for a
given drep - to prevent useless zero values being returned for possibly
hundreds of epochs for hundreds of dreps

## Description
As described above

## Which issue it fixes?
As described above

Co-authored-by: hodlonaut <[email protected]>

Spec adjustment following a fix to proposal list to use array
representation for withdrawals. couple of minor typos to existing
descriptions.

## Description
<!--- Describe your changes -->

## Which issue it fixes?
<!--- Link to issue: Closes #issue-number -->

Co-authored-by: hodlonaut <[email protected]>