Remove task context from python worker

[edoakes]

Why are these changes needed?

Removes duplicated state between the python and C++ workers. Also cleans up the serialization codepaths a bit.

Related issue number

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• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://ray.readthedocs.io/en/latest/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failure rates at https://ray-travis-tracker.herokuapp.com/.

[AmplabJenkins]

Can one of the admins verify this patch?

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[simon-mo]

Lgtm on serve changes. Thanks! I’ll leave @pcmoritz to review other parts.

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