Added discarding a future returned by get() into the docker store test.
Added discarding a future returned by get() into the docker store test.

This patch extends the `PullingSameImageSimultaneously` test
with discarding one of the futures returned by `store->get()`
and making sure that the pull still completes.

Review: https://reviews.apache.org/r/72793

reneploetz/mesos,reneploetz/mesos,dforsyth/mesos,Gilbert88/mesos,reneploetz/mesos,reneploetz/mesos,dforsyth/mesos,Gilbert88/mesos,Gilbert88/mesos,Gilbert88/mesos,abudnik/mesos,dforsyth/mesos,abudnik/mesos,reneploetz/mesos,dforsyth/mesos,reneploetz/mesos,Gilbert88/mesos,abudnik/mesos,abudnik/mesos,dforsyth/mesos,reneploetz/mesos,abudnik/mesos,dforsyth/mesos,abudnik/mesos,dforsyth/mesos,abudnik/mesos,Gilbert88/mesos,Gilbert88/mesos