issue #1: configuarable session-pool expire time and limit size

+ add session_pool::set_pool_time_limit(sec). default is 60sec
+ add session_pool::set_pool_size_limit(size). default is 0
  ( = unlimited ). Please note that
  - when pool-size reach limit, raise too_many_session_error
  - even if pool-size reach limit, existing idle sessions are not
    freed. these are freed only by expire-time