fix #500 to avoid potential hang and event loss

[ruitianzhong]

• add IfUsed() for worker to check if it is used.
• add Get() for routine that generates event before using the worker and Put() when after using the worker. They should not be called concurently(only one producer). So it panics if unexpected behavior happen.
• add clean up logic for ew.incoming and return only after both the ew.incoming is empty and ew is not used.

The reasoning for that is as follows.

When returned from ew.Close(), there are two possiblities:

1. no routine can touch it.
2. one routine can still touch ew because getWorkerByUUID()
   happen before ew.Close()

When no routine can touch it (i.e.,ew.IfUsed == false),
we just drain the ew.incoming and return.

When one routine can touch it (i.e.,ew.IfUsed == true), we ensure
that we only return after the routine can not touch it
(i.e.,ew.IfUsed == false). At this point, we can ensure that no
other routine will touch it and send events through the ew.incoming.
So, we return.

Because eworker has been deleted from workqueue after ew.Close()
(ordered by a workqueue lock), at this point, we can ensure that
no ew will not be touched even in the future. So the return is
safe.

Finally, to verify it, in ruitianzhong@e5bce57 , run go test -v ./pkg/event_processor/ -run TestHang , no hang happened.

Because the poc inject delay to reliably reproduce the hang, I do not add it as a unit test in bug-fix branch.

[cfc4n]

LGTM, thanks.