BUG/MEDIUM: checks: Dequeue checks on purge

BUG/MEDIUM: checks: Dequeue checks on purge

When tune.max-checks-per-thread is used, checks that should run are
queued, to avoid having too many checks running at the same time.
But if the check is about to be purged, because the server is being
deleted, we have to explicitly remove it from the queue as that memory is
about to be freed, otherwise it will cause a use-after-free.
Also, queued checks have not yet incremented th_ctx->running_checks, so
don't decrement it if we're queued.

This should be backported up to 3.0.