Expose Snapshot's SequenceNumber

Summary:
Requested here: https://www.facebook.com/groups/rocksdb.dev/permalink/705524519546065/

It might also help with mongo. I don't see a reason why we shouldn't expose this info.

Test Plan: make check

Reviewers: sdong, yhchiang, rven

Reviewed By: rven

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D32547

db/db_test.cc

@@ -7751,6 +7751,12 @@ class ModelDB: public DB {
   class ModelSnapshot : public Snapshot {
    public:
     KVMap map_;
+
+    virtual SequenceNumber GetSequenceNumber() const {
+      // no need to call this
+      assert(false);
+      return 0;
+    }
   };
 
   explicit ModelDB(const Options& options) : options_(options) {}

db/snapshot.h

@@ -20,6 +20,8 @@ class SnapshotImpl : public Snapshot {
  public:
   SequenceNumber number_;  // const after creation
 
+  virtual SequenceNumber GetSequenceNumber() const { return number_; }
+
  private:
   friend class SnapshotList;
 

include/rocksdb/db.h

@@ -64,6 +64,10 @@ static const int kMinorVersion = __ROCKSDB_MINOR__;
 // A Snapshot is an immutable object and can therefore be safely
 // accessed from multiple threads without any external synchronization.
 class Snapshot {
+ public:
+  // returns Snapshot's sequence number
+  virtual SequenceNumber GetSequenceNumber() const = 0;
+
  protected:
   virtual ~Snapshot();
 };