Add killing event and mark task as not running when killed

[dadgar]

Task "redis" is "dead"
Task Resources
CPU  Memory   Disk     IOPS  Addresses
500  256 MiB  300 MiB  0     db: 127.0.0.1:43968

Recent Events:
Time                   Type      Description
07/20/16 09:20:39 UTC  Killed    Task successfully killed
07/20/16 09:20:39 UTC  Killing   Sent interupt. Waiting 5s before force killing
07/20/16 09:20:35 UTC  Started   Task started by client
07/20/16 09:20:31 UTC  Received  Task received by client

[diptanu]

@dadgar With these changes does the Task Runner unblocks after calling the destroy on the driver? I think it will still block since the handle destroy and the subsequent call the to destroy method on the driver is called synchronously.

[dadgar]

@diptanu Yeah it blocks but that behavior is correct. We had bad locking at the alloc_runner level which was causing the issues you saw. This PR fixes them #1454

[diptanu]

@dadgar I see, that makes sense. LGTM

[github-actions]

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