Passing abstract Chain interface will let indexes run in separate
processes.
This commit does not change behavior in any way.
This is a partial backport of [[https://github.com/bitcoin/bitcoin/pull/25494 | core#25494]]
https://github.com/bitcoin/bitcoin/pull/25494/commits/33b4d48cfcdf145f49cb2283ac3e2936a4e23fff
Depends on D17704