availability-recovery: move cpu burners in blocking tasks (#7417)

* Move expensive computations to blocking thread

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* fix test

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* add internal error and fix dependent subystems

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* fmt

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* fix test fix

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* minor refactor and TODOs

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* Impl Feedback for Review

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* review feedback

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* More docs

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* add some example timings in comments

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

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Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>