fix(cluster): Don't miss keys when migrating slots (#3218)

In rare cases, the fuzzy cluster migration test detected missing keys.
It turns out that the missing keys were skipped at the source side due
to contention:
* The OnDbChange callback registered and got a `snapshot_id`
* It then blocked on a mutex, and could not add itself to the list of
  callbacks
* When the mutex was released, it registered, but it missed all changes
  that happened between registering (`snapshot_id`) and the moment it
  registered

src/server/db_slice.cc

@@ -1095,15 +1095,18 @@ void DbSlice::ExpireAllIfNeeded() {
 }
 
 uint64_t DbSlice::RegisterOnChange(ChangeCallback cb) {
-  uint64_t ver = NextVersion();
-
   // TODO rewrite this logic to be more clear
   // this mutex lock is needed to check that this method is not called simultaneously with
   // change_cb_ calls and journal_slice::change_cb_arr_ calls.
   // It can be unlocked anytime because DbSlice::RegisterOnChange
   // and journal_slice::RegisterOnChange calls without preemption
   std::lock_guard lk(cb_mu_);
+
+  uint64_t ver = NextVersion();
   change_cb_.emplace_back(ver, std::move(cb));
+  DCHECK(std::is_sorted(change_cb_.begin(), change_cb_.end(),
+                        [](auto& a, auto& b) { return a.first < b.first; }));
+
   return ver;
 }