fix(tables): serialize schema mutations to prevent parallel column clobber (#4812)

* Make workflow description nullable

* fix(tables): serialize schema mutations to prevent parallel column clobber

* fix(tables): load workflow outside schema lock; use DbOrTx for getTableById

* fix(tables): scale idle timeout in updateColumnType to avoid aborting large type changes

* fix(tables): skip stale remap types when workflowId changes concurrently

* fix(tables): scale idle timeout in updateColumnConstraints for large tables

Author: TheodoreSpeaks