switch to new sync/atomic helpers in go1.19 (#728)

These new helpers make the code a lot cleaner. I confirmed that the
simple helpers like `atomic.Int64` don't add any extra overhead as they
get inlined by the compiler. `atomic.Pointer` adds an extra method call
as it no longer gets inlined, but we aren't using these on the hot path
so it is probably okay.

cmd/nebula-service/main.go

@@ -13,7 +13,7 @@ import (
 
 // A version string that can be set with
 //
-//     -ldflags "-X main.Build=SOMEVERSION"
+//	-ldflags "-X main.Build=SOMEVERSION"
 //
 // at compile-time.
 var Build string