[assumeutxo] document and enforce incompatability with -chronik

Summary:
There are currently multiple issues with combining Chronik and assumeutxo:

• Chronik crashes on indexing mempool transactions that spend outputs that are not yet indexed
• Chronik likely cannot handle a BlockDisconnected notification from the assumevalid chain while indexing the ibd chain

In any case, a Chronik instance is not really usable until it has fully indexed the entire blockchain, so fixing these issues is not a priority and should not delay a release with assumeutxo. Users can enable chronik after the background sync is completed, as demonstrated by the added functional test.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18079