Switch debugger to sockets and support unicode

[pcmoritz]

Why are these changes needed?

This PR switches the Ray debugger client from telnet to sockets so we can support unicode characters in the source code.

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://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
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  • This PR is not tested :(

[richardliaw]

nice! does this mean we can add ipdb next :)

[pcmoritz]

Yeah, first we need to get tab completion working (which is actually supported by both pdb and ipdb).

Since the tab completion information is only available in the remote process (naturally), to make it available in the client, we need to both flush the stdin of the client as well as the socket that connects it to the remote process as soon as we encounter a tab. This turns out to be more involved than anticipated because there is some buffering in place for both at the moment, I'll keep working on it tomorrow :)