When setting a new strategy, the charges of old tracker is transferred
to the new one.
The problem is that we're not reverting changes if exception is
triggered before the new strategy is successfully set.
To fix this exception safety issue, let's copy the charges instead
of moving them. If exception is triggered, the old tracker is still
the one used and remain intact.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
244efddb22