fix buffer configurations

fix buffer configurations such as:
 * specified in out_webhdfs.rb
   * buffer_type memory
   * buffer_path must be specified for `buffer_type file`
 * TimeSlicedOutput default options
   * flush_interval nil
   * buffer_chunk_limit 256m
 * buf_memory's default options:
   * buffer_queue_limit 64

Are these descriptions copy&past-ed from out_exec_filter's documents?
(originally `flush_interval 1s`)