rdb_compat: rdb_compat reuses connection by sequel's connection pooling. max_connections is 1.
rdb_compat: keepalive timeout enabled. default timeout is 10 sec.
rdb_compat: added :use_connection_pooling flag. users can configure use of connection pooling to access database. the flag is, by default, false. if config has :sslca value, it is true.
rdb_compat: use config.fetch method instead of config[:use_connection_pooling]
rdb_compat: moved disconnect method call for conncetion_pooling to ensure block.
rdb_compat: minor changed