Fix performance regression in Get() for block-based tables (#3953)

Summary:
This fixes a regression in one of myrocks regression tests (readwhilewriting), introduced in 8bf555f487

This PR changes two lines of code: one of them actually fixes the observed regression, the other is a mostly unrelated small fix that I'm piggy-backing here. EDIT: Nevermind, it fixes one line. More details in inline comments.
Closes https://github.com/facebook/rocksdb/pull/3953

Differential Revision: D8270664

Pulled By: al13n321

fbshipit-source-id: a7d91e196807d1e816551591257c700f70e4ccac

table/block.h

@@ -259,7 +259,7 @@ class BlockIter final : public InternalIterator {
   }
 
   // Makes Valid() return false, status() return `s`, and Seek()/Prev()/etc do
-  // nothing.
+  // nothing. Calls cleanup functions.
   void Invalidate(Status s) {
     // Assert that the BlockIter is never deleted while Pinning is Enabled.
     assert(!pinned_iters_mgr_ ||
@@ -269,6 +269,9 @@ class BlockIter final : public InternalIterator {
     current_ = restarts_;
     status_ = s;
 
+    // Call cleanup callbacks.
+    Cleanable::Reset();
+
     // Clear prev entries cache.
     prev_entries_keys_buff_.clear();
     prev_entries_.clear();