alloc_runner: stop sidecar tasks last

[DerekStrickland]

Closes #10340

This PR addresses a defect where sidecar tasks are stopped prior to their associated main task being successfully stopped. This can create problems in scenarios where the main task can take a long time to shut down, but still need their sidecars to be operational while they finish their shutdown task (e.g. Consul Connect proxy).

This fix includes the following changes:

• Adds the IsSidecarTask helper method.
• Refactors other helpers to use IsSidecarTask.
• Updates the doc string on the hasSidecarTasks method to correctly describe the behavior of the function.
• Modifies the second kill loop in allocRunner.killTasks to skip shutting down sidecar tasks. This allows the existing logic in allocHandler.handleTaskStateUpdates to shutdown sidecars after the main task has exited as it was designed to do.
• Renames a variable to avoid package import collision.

[lgfa29]

Minor suggestion about the log message.

I was also looking at the existing test suite, but it doesn't seem like we have any timing based tests. One thing that I thought was to check the task event order in TestAllocRunner_Lifecycle_Poststart but I don't know how reliable that would be.

[DerekStrickland]

@lgfa29

Thanks for catching the test omission. I'll take a pass at adding a specific test for this.

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