feat: Add 2 options to limit thread count (#2011)

There are 2 options stats_dump_period_sec and stats_persist_period_sec in
RocksDB to adjust the period to dump/persist stat, each RocksDB instance
creates 2 dependent threads for this aim.

A replica server is possible to serve thousands of replicas, it means
thousands of RocksDB instances are possible to be running in one process,
then thousands of threads will be created will may consume much resouce.

This patch introduces 2 options, then it's possible to disable the threads
by setting them to 0.

src/server/pegasus_server_impl_init.cpp

@@ -568,6 +568,14 @@ DSN_DEFINE_string(pegasus.server,
 DSN_DEFINE_validator(rocksdb_filter_type, [](const char *value) -> bool {
     return dsn::utils::equals(value, "common") || dsn::utils::equals(value, "prefix");
 });
+DSN_DEFINE_uint64(pegasus.server,
+                  stats_dump_period_sec,
+                  600, // 600 is the default value in RocksDB.
+                  "If not zero, dump rocksdb.stats to RocksDB LOG every stats_dump_period_sec");
+DSN_DEFINE_uint64(pegasus.server,
+                  stats_persist_period_sec,
+                  600, // 600 is the default value in RocksDB.
+                  "If not zero, dump rocksdb.stats to RocksDB every stats_persist_period_sec");
 
 namespace dsn {
 namespace replication {
@@ -663,6 +671,8 @@ pegasus_server_impl::pegasus_server_impl(dsn::replication::replica *r)
     _db_opts.listeners.emplace_back(new pegasus_event_listener(this));
     _db_opts.max_background_flushes = FLAGS_rocksdb_max_background_flushes;
     _db_opts.max_background_compactions = FLAGS_rocksdb_max_background_compactions;
+    _db_opts.stats_dump_period_sec = FLAGS_stats_dump_period_sec;
+    _db_opts.stats_persist_period_sec = FLAGS_stats_persist_period_sec;
     // init rocksdb::ColumnFamilyOptions for data column family
     _data_cf_opts.write_buffer_size = FLAGS_rocksdb_write_buffer_size;
     _data_cf_opts.max_write_buffer_number = FLAGS_rocksdb_max_write_buffer_number;