Linked against the executor in the agent to ensure ordered message delivery.
Linked against the executor in the agent to ensure ordered message delivery.

Previously, we did not `link` against the executor `PID` while
(re)-registering. This might lead to libprocess creating ephemeral
sockets everytime a `send` was invoked. This was leading to races
where messages might appear on the Executor out of order. This change
does a `link` on the executor PID to ensure ordered message delivery.

Review: https://reviews.apache.org/r/40660

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