[backport PR12653] Allow to optional specify the directory for the blocks storage

Summary:
from: https://github.com/bitcoin/bitcoin/pull/12653/files
a1926362e -blocksdir: keep blockindex leveldb database in datadir
f38e4fdb0 QA: Add -blocksdir test
386a6b62a Allow to optional specify the directory for the blocks storage

also includes bugfix PR15124 (Ben Woosley)
ba8c8b2227 Fail if either disk space check fails

Test Plan:
make check
test_runner.py

• manual testing on testnet

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Maniphest Tasks: T636

Differential Revision: https://reviews.bitcoinabc.org/D2939