db_bench was not correctly initializing the value for delete_obsolete_files_period_micros option.

Summary:
The parameter delete_obsolete_files_period_micros controls the
periodicity of deleting obsolete files. db_bench was reading in
this parameter intoa local variable called 'l' but was incorrectly
using another local variable called 'n' while setting it in the
db.options data structure.
This patch also logs the value of delete_obsolete_files_period_micros
in the LOG file at db startup time.

I am hoping that this will improve the overall write throughput drastically.

Test Plan: run db_bench

Reviewers: MarkCallaghan, heyongqiang

Reviewed By: MarkCallaghan

Differential Revision: https://reviews.facebook.net/D6099

db/db_bench.cc

@@ -1299,7 +1299,7 @@ int main(int argc, char** argv) {
       FLAGS_disable_seek_compaction = n;
     } else if (sscanf(argv[i], "--delete_obsolete_files_period_micros=%ld%c",
                       &l, &junk) == 1) {
-      FLAGS_delete_obsolete_files_period_micros = n;
+      FLAGS_delete_obsolete_files_period_micros = l;
     } else if (sscanf(argv[i], "--stats_interval=%d%c", &n, &junk) == 1 &&
                n >= 0 && n < 2000000000) {
       FLAGS_stats_interval = n;

util/options.cc

@@ -93,6 +93,8 @@ Options::Dump(
         db_log_dir.c_str());
     Log(log,"                Options.disable_seek_compaction: %d",
         disable_seek_compaction);
+    Log(log,"    Options.delete_obsolete_files_period_micros: %ld",
+        delete_obsolete_files_period_micros);
 }   // Options::Dump