Improve timeouts handling to make sure Node processes are not hanging

[katafrakt]

This is a followup to #102 with improvements suggested there in the closing comment.

This improves how timeouts are handles by:

• Removing the race condition from where timeout originates - now it's always coming from GenServer.call
• Killing the worker process on timeout, so the next time the worker is checked out from the pool, it for sure has a fresh and working timeout
• Replacing returning strings in error tuples with an atom

[s3cur3]

This is a lovely simplification, and way better than my initial attempt to standardize the timeout handling in #102. 🙌

[jtippett]

+1 — we hit this exact case_clause crash in production after upgrading to 3.1.4. Our Node SSR worker SIGABRT'd (exit 134) and the worker GenServer crashed with CaseClauseError({:error, {:exit, 134}}) because get_response/4 started returning that shape in #105 but handle_call/3's case clause was never updated to match it.

The catch-all {:error, _} = error -> {:reply, error, state} here is the right fix. We've pinned to this branch via {:nodejs, github: "katafrakt/elixir-nodejs", ref: "..."} to keep production working — would love to see this merged so we can switch back to a hex release. Happy to test if that helps.