server-side changes for SET distribution

Summary:
In this diff we add:
1. A callback to process SET requests from DL. The callback takes a `writeTimestamp` argument that will be passed down to the command handler (in the next diff). We will compare this writeTimestamp with the existing item's creation time and discard the write if the writeTimestamp is older than the item's creation time.
2. Refactor Bucket Manager to take two callbacks - one for delete, second for writes. Depending on the payload in WH update, we call the relevant callback.

In the next diffs we will add the logic in the MemcachedCommandHandler to accommodate the passed in `writeTimestamp`, as well as security logic.

Reviewed By: stuclar

Differential Revision: D57074727

fbshipit-source-id: 58c499fb9cc88e0fe0951312c362ffc70973414f

mcrouter/lib/carbon/RequestCommon.h

@@ -102,6 +102,10 @@ class RequestCommon : public MessageCommon {
     sourceIpAddr_ = sourceIpAddr;
   }
 
+  void setWriteTimestamp(uint64_t writeTimestamp) noexcept {
+    writeTimestamp_ = writeTimestamp;
+  }
+
  protected:
   void markBufferAsDirty() {
     serializedBuffer_ = nullptr;
@@ -115,6 +119,8 @@ class RequestCommon : public MessageCommon {
   std::optional<std::string> clientIdentifier_;
   // Source ip address.
   std::optional<folly::IPAddress> sourceIpAddr_;
+  // write timestamp
+  std::optional<uint64_t> writeTimestamp_;
 };
 
 } // namespace carbon