SessionContext execute broken

**Describe the bug**
The `execute` function of `SessionContext` creates a new `TaskContext` with an empty `runtime_env` which wipes out any `ObjectStore`s among many other things.

**To Reproduce**
Use the `SessionContext` the `execute` on a `PhysicalPlan` that queries a table in a registered `ObjectStore`.

**Expected behavior**
Execute uses existing `SessionState` from `SessionContext`.