snapshot_period remains unused.

`snapshot_period` in `CommonParams` was added to determine the snapshot period. However, because of term specific data like `intermediate_reward`, the current snapshot period is fixed at 1 term.
There are two options:
1. Get rid of it from `CommonParams`
2. Modify its usage to designate the snapshot period as `snapshot_period` terms. It may accompany field renaming.

Start by locating CommonParams and every use of snapshot_period, then inspect how intermediate_reward and the current one-term snapshot behavior are handled. Determine whether the intended outcome is removing the field or making it control a multi-term snapshot period, including any field renaming. Done means the chosen behavior is specified and consistently implemented, with relevant tests updated or added.

Written by the indexing model from the issue text.