Access miner fund destinations by reference in getblocktemplate

Summary: No need to copy. This patch makes it const too.

Test Plan:

ninja && ./test/functional/test_runner.py abc_feature_minerfund abc_mining_basic

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12984