kvserver: sync txn record cleanup can be cancelled prematurely

[erikgrinaker]

As described in #60585 (comment), when IntentResolver.CleanupTxnIntentsAsync() is called with allowSyncProcessing=true, the cleanup task can be run synchronously and using the caller's context. However, this cleanup task spawns off another async task to clean up the transaction record:

cockroach/pkg/kv/kvserver/intentresolver/intent_resolver.go

Lines 757 to 762 in a4b7a43

	// Run transaction record GC outside of ir.sem.
	return ir.stopper.RunAsyncTask(
	ctx,
	"storage.IntentResolver: cleanup txn records",
	func(ctx context.Context) {
	err := ir.gcTxnRecord(ctx, rangeID, txn)

In the synchronous case, this cleanup task will be connected to the caller's context, but runs async. As such, the call will likely return to the caller before gcTxnRecord() completes, and the caller may the cancel the context (due to e.g. defer cancel() or disconnection) which will cancel gcTxnRecord() as well.

[nvanbenschoten]

This is a nice find. @lunevalex should be aware of it when touching this code in #59881. In that change, I think we'll want to pass a Background context to SendAsync so that we don't hit the same kind of issue as the one described here.