refactor: relax closure constraints from FnMut to FnOnce

[gomnitrix]

This addresses #231 by relaxing the closure trait bounds from FnMut to FnOnce in fetch_update and update_fetch methods across all transaction types.

Also removed inaccurate documentation in single_writer/keyspace.rs stating that closures may be called multiple times on retries (only true for the optimistic variant with retry logic).