v0.1.2: Performance Improvments & Bug Fixes

Highlights

• improve performance about timer and threadpool
• threadpool.first_waiting_executor is deprecated. Please use threadpool.remove_avaliable_executor instead
• threadpool.create_executor does not return the descriptor
• set_timers now return the timers you set
• set_timeout, set_repeat now return the timer
• fix: threadpool may unexpectedly run function in dead executor