Use named handler export rather than default export for lambdas

[ajmacd]

The lambdas were erroring with:

{
    "errorType": "Runtime.HandlerNotFound",
    "errorMessage": "index.handler is undefined or not exported"
}

AFAICT, the handler function needs to be a named, and not default, export.

Furthermore, it appeared that, in some cases at least, the startstop lambda could exit before the network request to the ECSClient completed. This also now awaits the response before exiting the lambda.

Finally, this causes the status lambda to always return some value.

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[ajmacd]

@RyoshiKayo interested in your perspective here if you get a chance to take a look, as the (apparently) breaking changes were due to the improvements you made earlier this year. Curious to know why you weren't hitting the issue.