* Move optimise out of RunAllMigrations
* Separate read and write database connections
* Enforce readonly connection constraint
* Fix migrations not using tx
* #5155 - allow setting cache size from environment
* Document new environment variable
* Make read-only operations use WithReadTxn
* Allow one database write thread
* Add unit test for concurrent transactions
* Perform some actions after commit to release txn
* Suppress some errors from cancelled context
