Add extra statistic to track the number of signature validation operations (#1289)

Add extra statistic to track the number of signature validation operations (#1289)

* Add extra statistic to track the number of signature validation operations performed by the validator module

* Move validation operation statistic to mesh as suggested

* Fix NULL pointer dereference in case the mesh is not used (and is `NULL`)

Co-authored-by: Wouter Wijngaards <wcawijngaards@users.noreply.github.com>
* Fix NULL pointer dereference on qstate and qstate->env in unit test situation

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Co-authored-by: Wouter Wijngaards <wcawijngaards@users.noreply.github.com>