Handled hanging docker `stop`, `inspect` commands in docker executor.
Handled hanging docker `stop`, `inspect` commands in docker executor.

Previosly, if `docker inspect` command hanged, the docker container
ended up in an unkillable state. This patch adds a timeout for inspect
command after receiving `killTask` analogically to `reaped` handler.
In addition we've added a timeout for `docker stop` command. If docker
`stop` or `inspect` command times out, we discard the related future,
thus the docker library kills previously spawned docker cli subprocess.
As a result, a scheduler can retry `killTask` operation to handle
nasty docker bugs that lead to hanging docker cli.

Review: https://reviews.apache.org/r/65713/

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