Sanity check on Open

Summary:
Everytime a client opens a DB, we do a sanity check that:
* checks the existance of all the necessary files
* verifies that file sizes are correct

Some of the code was stolen from https://reviews.facebook.net/D16935

Test Plan: added a unit test

Reviewers: dhruba, haobo, sdong

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17097

db/db_impl.h

@@ -94,6 +94,10 @@ class DBImpl : public DB {
   virtual void GetLiveFilesMetaData(
     std::vector<LiveFileMetaData> *metadata);
 
+  // checks if all live files exist on file system and that their file sizes
+  // match to our in-memory records
+  virtual Status CheckConsistency();
+
   virtual Status GetDbIdentity(std::string& identity);
 
   Status RunManualCompaction(int input_level,