make all DB::Get overloads virtual

Summary:
some fbcode services override it, we need to keep it virtual.

original change: #1756
Closes https://github.com/facebook/rocksdb/pull/2065

Differential Revision: D4808123

Pulled By: ajkr

fbshipit-source-id: 5eaeea7

include/rocksdb/db.h

@@ -280,7 +280,7 @@ class DB {
   // a status for which Status::IsNotFound() returns true.
   //
   // May return some other Status on an error.
-  inline Status Get(const ReadOptions& options,
+  virtual inline Status Get(const ReadOptions& options,
                             ColumnFamilyHandle* column_family, const Slice& key,
                             std::string* value) {
     assert(value != nullptr);

utilities/document/document_db.cc

@@ -1037,6 +1037,7 @@ class DocumentDBImpl : public DocumentDB {
   }
 
   // RocksDB functions
+  using DB::Get;
   virtual Status Get(const ReadOptions& options,
                      ColumnFamilyHandle* column_family, const Slice& key,
                      PinnableSlice* value) override {