Print DB pointer when opening a DB

Summary: Having a pointer for DB will be helpful to debug when GDB or working on a dump. If the client process doesn't have any thread actively working on RocksDB, it can be hard to find out.

Test Plan: make all check

Reviewers: rven, yhchiang, igor

Reviewed By: igor

Subscribers: yoshinorim, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D33159

db/db_impl.cc

@@ -3853,6 +3853,8 @@ Status DB::Open(const DBOptions& db_options, const std::string& dbname,
 
   if (s.ok()) {
     impl->opened_successfully_ = true;
+    Log(InfoLogLevel::INFO_LEVEL, impl->db_options_.info_log, "DB pointer %p",
+        impl);
     *dbptr = impl;
   } else {
     for (auto* h : *handles) {