There are quite a few variables in the code that are no longer
(or perhaps never were) in use. These were discovered by enabling
compiler warnings for unused variables and cleaning them up.
In most cases where the unused variables were the result
of a function call the call was left but the variable
assignment removed, unless it was obvious that it was
a simple getter with no side effects.
This is technically a record encrypted in two pieces,
so the iv needs to be different.
Some backward compatibility is added to read data written
by existing code, but new data is written with the new code.
This will avoid careless forkers polluting the shared database
even if they make their own chain. They'll then automatically
start using another subdb, and any key-reusing fork of those
forks will reuse their subdbs.
This maps key images to rings, so that different forks can reuse
the rings by key image. This avoids revealing the real inputs like
would happen if two forks spent the same outputs with different
rings. This database is meant to be shared with all Monero forks
which don't bother making a new chain, putting users' privacy at
risk in the process. It is placed in a shared data directory by
default ($HOME/.shared-ringdb on UNIX like systems). You may
use --shared-ringdb-dir to override this location, and should
then do so for all Monero forks for them to share the database.
