Update GetCoins API to use the Coin abstraction.

Summary:
This is a continuation of the work started in D342 and D343 . Before changing the underlying storage, we need to miigrate the API.

Depends on D343

Test Plan:

make check
../qa/pull-tester/rpc-tests.py

Reviewers: freetrader, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D346