Better management of storage merging in parallel-executor

// 0. Make a transaction with the DA changes in a Arc used by everyone afterwards
// 1. All contract changes in 1 changes
// 2. Create a storage transaction on top of the storage transaction that have the DA Changes
// (created at the start of run function and re-used by all executions)
// 3. Execute the transactions on top of this storage transaction

Start by locating the parallel-executor run function and the storage-transaction code it uses. Trace how DA changes and contract changes are currently handled across executions. Done means the three listed transaction-management steps are implemented and the storage merging behavior is covered by the project's relevant tests.

Written by the indexing model from the issue text.