use ChainstateManager to initialize chainstate

Summary:

This allows us to easily initialize multiple chainstates on startup in
future commits. It retires the g_chainstate global in lieu of
g_chainman.

Partial backport (3/5) of core PR17737:
https://github.com/bitcoin/bitcoin/pull/17737/commits/4ae29f5f0c5117032debb722d7049664fdceeae8

Includes a fix to qt/test/apptests.cpp from the next commit to unbreak
the bitcoin-qt tests:
https://github.com/bitcoin/bitcoin/pull/17737/commits/2b081c4568e8019886fdb0f2a57babc73d7487f7#diff-41e328d9ba5879d2bd026a700743339b5b300b9bee50844795ef9e27f60bdfb8R85

Depends on D8538.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8539