Add an upwards reference to chainstate instances to the owning
ChainstateManager. This is necessary because there are a number
of `this_chainstate == chainman.ActiveChainstate()` checks that
will happen (as a result of assumeutxo) in functions that otherwise
don't have an easily-accessible reference to the chainstate's
ChainManager.
This is a backport of core#21526 [1/12]
https://github.com/bitcoin/bitcoin/pull/21526/commits/9f6bb539359b98d5b39482ab8a28a68608f0c645