Use cPickle to pass messages between slaves and the master.
Use cPickle to pass messages between slaves and the master.

Instead of using raw strings, we should use cPickle to pass messages betwen
the slaves and the master. In future, this will allow us to attach additional
information to the messages such as what packages failed.

BUG=chromium-os:32249
TEST=Verify with remote trybot that it still reads the right message from
     itself when a build fails.

Change-Id: I7736b2298d10a01dd6960e77dbe84f557da6b3d1
Reviewed-on: https://gerrit.chromium.org/gerrit/27422
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Commit-Ready: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,zhang0137/chromite,coreos/chromite,zhang0137/chromite,coreos/chromite,chadversary/chromiumos.chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite