This implements automatic reindexing of old database, turned on by
default. When an old database is detected and `auto_reindex` is turned
on the database will be destroyed so that `electrs` can re-sync. The
user can still turn this off to check db format.
The help message says "or inconsistent" - this is not implemented but we
could in the future and it seems reasonable to explicitly state that
this option will control that thing as well.
Note: 0.8.x versions of `electrs` didn't contain format information -
so we check if the database is empty and assuming it's legacy if it's
not empty but without format information.
The code is also restructured a bit and a few tests are added.
