The PartitionManager needs to track the number of in-flight transactions
for each partition. Partitions without any in-flight transactions are stopped
as idle.

The tracking is currently done by injecting a factory method into TransactionManager,
which is used to associate a finalizer with each Transaction. While this is now done
in TransactionManager, this is not really its responsibility as it doesn't need this
information at all. This makes TransactionManager more complex as it has to keep
calling the finalizer in the right location and has to care about details it doesn't
need. The finalizer should always be run after the transaction's Commit or Rollback
call has finished.

This commit improves the situation by instead wrapping the Transaction returned from
TransactionManager and running the finalizer logic once Commit or Rollback is called
on the wrapper. This makes the call flow clearer as it doesn't go back and forth
between types. It also keeps unneeded details out from the TransactionManager which
is already complex enough on its own.

The wrapper type finalizableTransaction is private as it won't be used directly in
other packages. This is different from the other types as the Transaction itself
is public. We'll fix this later by making it and other types private which are now
unnecessarily public. The `storagemgr` package will handle the lifecycle of the
transactions as well when we eventually integrate it in Gitaly. We'll only expose
a minimal interface publicly.