Commit precondition on cells not changing.

Some write transactions only want to commit if some other fields, such as those read, have no changed concurrently. For example, if you are computing a column y based on a column x, you don't want to commit the new values for y if column x has concurrently changed, since that would place stale values.

Add a precondition onto the commit. Should pass field ids to choose columns, and RowAddressTreemap to specify which rows. There could potentially be multiple pre-conditions. Support for passing row ids (stable row ids) is left for a future change.

let rows_covered = RowAddressTreeMap::from(100..200);
CommitBuilder::new(dataset)
   .with_precondition(Precondition::cells_unchanged([3, 4], rows_covered))
   .execute(transaction)
   .await?

Start by tracing CommitBuilder::execute and the transaction commit path, then inspect Precondition and RowAddressTreeMap usage. The work is complete when cells_unchanged can accept field IDs and covered rows, supports multiple commit preconditions, and prevents committing when those cells changed concurrently.