CompactedDB: log using the correct info_log

Summary:
info_log from supplied Options can be nullptr. Using the one from
db_impl. Also call flush after that since no more loggging will happen
and LOG can contain partial output

Test Plan: verified with db_bench

Reviewers: igor, yhchiang, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D24183

db/db_impl_readonly.cc

@@ -14,7 +14,8 @@ namespace rocksdb {
 DBImplReadOnly::DBImplReadOnly(const DBOptions& db_options,
                                const std::string& dbname)
     : DBImpl(db_options, dbname) {
-  Log(db_options_.info_log, "Opening the db in read only mode");
+  Log(INFO_LEVEL, db_options_.info_log, "Opening the db in read only mode");
+  LogFlush(db_options_.info_log);
 }
 
 DBImplReadOnly::~DBImplReadOnly() {

utilities/compacted_db/compacted_db_impl.cc

@@ -144,8 +144,10 @@ Status CompactedDBImpl::Open(const Options& options,
   std::unique_ptr<CompactedDBImpl> db(new CompactedDBImpl(db_options, dbname));
   Status s = db->Init(options);
   if (s.ok()) {
+    Log(INFO_LEVEL, db->db_options_.info_log,
+        "Opened the db as fully compacted mode");
+    LogFlush(db->db_options_.info_log);
     *dbptr = db.release();
-    Log(options.info_log, "Opened the db as fully compacted mode");
   }
   return s;
 }