DbTxnManager comes with DbLockManager implementation that doesn't support
explicit lock/unlock functionality.
It was designed to be used only for ACID tables, however, due to code refactor
challenges is reused for external tables as well. This comes with few drawbacks
like suboptimal behavior (opens a txn for an ext table) and reduced
functionality.
That's something we plan to address in a future. For now you can configure a
dedicated HS2 instance with DummyTxnManager to have explicit locking
functionality for ext tables.
