Consistency Check Function

Summary: Added a function/command to check the consistency of live files' meta data

Test Plan:
Manual test (size mismatch, file not exist).
Command test script.

Reviewers: haobo

Reviewed By: haobo

CC: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D16935

util/ldb_tool.cc

@@ -53,6 +53,7 @@ public:
     DeleteCommand::Help(ret);
     DBQuerierCommand::Help(ret);
     ApproxSizeCommand::Help(ret);
+    CheckConsistencyCommand::Help(ret);
 
     ret.append("\n\n");
     ret.append("Admin Commands:\n");