Change to code for trimmed memtable history is to released outside DB mutex (#8530)

Summary:
Currently, the code shows that we delete memtables immedately after it is trimmed from history. Although it should never happen as the super version still holds the memtable, which is only switched after it, it feels a good practice not to do it, but use clean it up in the standard way: put it to WriteContext and clean it after DB mutex.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8530

Test Plan: Run all existing tests.

Reviewed By: ajkr

Differential Revision: D29703410

fbshipit-source-id: 21d8068ac6377de4b6fa7a89697195742659fde4

db/db_impl/db_impl_write.cc

@@ -1651,12 +1651,7 @@ Status DBImpl::TrimMemtableHistory(WriteContext* context) {
   for (auto& cfd : cfds) {
     autovector<MemTable*> to_delete;
     bool trimmed = cfd->imm()->TrimHistory(
-        &to_delete, cfd->mem()->ApproximateMemoryUsage());
-    if (!to_delete.empty()) {
-      for (auto m : to_delete) {
-        delete m;
-      }
-    }
+        &context->memtables_to_free_, cfd->mem()->ApproximateMemoryUsage());
     if (trimmed) {
       context->superversion_context.NewSuperVersion();
       assert(context->superversion_context.new_superversion.get() != nullptr);