Removed const fields in copyable classes (#5095)

Summary:
This fixed the compile error in Clang-8:
```
error: explicitly defaulted copy assignment operator is implicitly deleted [-Werror,-Wdefaulted-function-deleted]
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5095

Differential Revision: D14811961

Pulled By: riversand963

fbshipit-source-id: d935d1f85a4e8694dca10033fb5af92d8777eca0

utilities/persistent_cache/block_cache_tier.h

@@ -100,7 +100,7 @@ class BlockCacheTier : public PersistentCacheTier {
 
     std::string key_;
     std::string data_;
-    const bool signal_ = false;  // signal to request processing thread to exit
+    bool signal_ = false;  // signal to request processing thread to exit
   };
 
   // entry point for insert thread

utilities/persistent_cache/block_cache_tier_file.h

@@ -266,7 +266,7 @@ class ThreadedWriter : public Writer {
     size_t Size() const { return sizeof(IO); }
 
     WritableFile* file_ = nullptr;     // File to write to
-    CacheWriteBuffer* const buf_ = nullptr;  // buffer to write
+    CacheWriteBuffer* buf_ = nullptr;  // buffer to write
     uint64_t file_off_ = 0;            // file offset
     bool signal_ = false;              // signal to exit thread loop
     std::function<void()> callback_;   // Callback on completion