[Enhancement]: optimize task manager

[chris-sun-star]

on branch 2.0.x_dev
every time a task is submitted, task manager will create a go routine to execute it, maybe it's a better way to use a coroutine pool, and add timeout for task execution

[powerfooI]

PR #95 fixes the possible concurrent writes to the task and taskResult maps.

[powerfooI]

If number of wrong tasks bursts the operator manager will panic, because there is no throttling logic in task manager and task goroutines can be created infinitely.

Maybe a throttler should be added into TaskManager struct.