Fix column family seconds_up accounting

Summary:
`cf_stats_snapshot_.seconds_up` appears to be never updated, unlike `db_stats_snapshot_.seconds_up`, which is updated here: https://github.com/facebook/rocksdb/blob/master/db/internal_stats.cc#L883

This leads to wrong information in the log, for example:
```
** Compaction Stats [default] **

....

Uptime(secs): 85591.2 total, 85591.2 interval
```

Even though DB's interval is correctly logged as 60 seconds:
```
** DB Stats **
Uptime(secs): 85591.2 total, 637.8 interval
```
Closes https://github.com/facebook/rocksdb/pull/2338

Differential Revision: D5114131

Pulled By: sagar0

fbshipit-source-id: 85243a38213236ccbb601a7f7aaa8865eaa8083c

db/internal_stats.cc

@@ -1139,6 +1139,7 @@ void InternalStats::DumpCFStatsNoFileHistogram(std::string* value) {
            total_stall_count - cf_stats_snapshot_.stall_count);
   value->append(buf);
 
+  cf_stats_snapshot_.seconds_up = seconds_up;
   cf_stats_snapshot_.ingest_bytes_flush = flush_ingest;
   cf_stats_snapshot_.ingest_bytes_addfile = add_file_ingest;
   cf_stats_snapshot_.ingest_files_addfile = ingest_files_addfile;