Rename ExecutionMode to IsolationStrategy (#1932)

* Rename ExecutionMode to IsolationStrategy

Execution mode is too generic name and can imply a lot of different
aspects of execution. The notion of isolation better describes the
meant aspect.

And while I am at it, I also renamed mode -> strategy cause it seems a
bit more appropriate, although that is way more subjective.

* Fix compilation in wasm_executor tests.

* Add a comment to IsolationStrategy

* Update comments on IsolationStrategy

* Update node/core/candidate-validation/src/lib.rs

Co-authored-by: Bastian Köcher <[email protected]>

* Accomodate the point on interruption

* Update parachain/src/wasm_executor/mod.rs

Co-authored-by: Andronik Ordian <[email protected]>

* Naming nits

Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: Andronik Ordian <[email protected]>