colexecargs: deeply reset monitor registry on Reset

Given some recent issues we've had with leaks around memory monitors, it
seems prudent to deeply reset the slice of monitors that is stored in
the monitor registry object and is reused. Note that we haven't seen any
evidence of leaks related to this registry, but this change might make
things easier for GC, so seems worth it.

Release note: None

pkg/sql/colexec/colexecargs/monitor_registry.go

@@ -246,8 +246,12 @@ func (r *MonitorRegistry) Close(ctx context.Context) {
 
 // Reset prepares the registry for reuse.
 func (r *MonitorRegistry) Reset() {
-	// There is no need to deeply reset the memory monitoring infra slices
-	// because these objects are very tiny in the grand scheme of things.
+	for i := range r.accounts {
+		r.accounts[i] = nil
+	}
+	for i := range r.monitors {
+		r.monitors[i] = nil
+	}
 	r.accounts = r.accounts[:0]
 	r.monitors = r.monitors[:0]
 }