[rpc] Add initialblockdownload to getblockchaininfo

Summary:
PR11258 backport https://github.com/bitcoin/bitcoin/pull/11258/files

Just first commit; I'm omitting the second commit which just does
inapplicable reformatting. Also, modified to use pushKV.

Includes release notes copied from Core release-notes-0.16.0.md

Test Plan:
make check
test_runner.py

Reviewers: jasonbcox, #bitcoin_abc, deadalnix, Fabien

Reviewed By: jasonbcox, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3101