Executor versioning [#3152]

• Closes #3152 (closed) and fixes #3153 (closed).
• Adds an executor version field to the system ledger.
• Adds a transaction that updates the executor version and is propagated out from the DN to the BVNs.
• Adds an executor version field to the global values, with the same mechanics as other globals.
• Updates the database to track how many times a value has been changed (in memory, not stored).
• Logs an error if multiple batches concurrently change the same value.

Changelog: feature