Master doesn't remove task from pending when it is invalid

In `Master::_accept()` there are cases when a task is not launched (e.g, agent disconnected, agent removed or task group is invalid). Doesn't look like we make sure the task is removed from `framework->pending` in this case. In addition to the memory leak reconciliation will result in a scheduler thinking the task is still in TASK_STAGING. Same for operators who are looking at operator endpoints.

Note that pending task not being removed when agent is disconnected/removed is a long standing bug. We should fix it in all the supported versions.