Use query param to intake API to signal that function completed

[estolfo]

This PR adds a change to signal on a channel when it receives an intake request from an agent containing the query param "flushed=true".
We can change the name of the query param, if that's not clear enough.

As a fallback, it also signals if it receives a runtimeDone event from the logsapi.

There are now three channels used to signal in main. The AgentDoneSignal signals that there was an intake request received with flushed=true, the runtimeDoneSignal channel signals that a runtimeDone event was received via the Logs API. The funcDone channel is closed if a signal is received on AgentDoneSignal or runtimeDoneSignal.
Closing that channel signals to the go routines posting apm data and processing logs API events that they can return.

Resolves #62

[ghost]

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• Start Time: 2021-12-01T14:08:10.430+0000

• Duration: 7 min 40 sec

• Commit: 5b2295b

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[astorm]

Overall this looks simple and solid. I see we have a new FuncDone property on the extension package that we're using as a signal only channel, and when this channel receives a message it indicates the function is done with execution.

We send a signal to this channel whenever we see a request to /intake/v2/events that includes a flushed=true query string parameter.

This should work. However, when I tested this against a real agent without the query param implemented the extension crashed and interfered with a Node.js Lambda function (causing it to return null)

However we coordinate with the agents to get this query param in we should (probably?) ensure that lambda functions with older agents attached don't crash. (I've added a discussion topic to the next planning meeting)

[estolfo]

Thanks for the review, @astorm. I'll try to reproduce the error and report back.

[astorm]

Approving 👍

I ran a manual smoke test and the fallback code is working -- data's sent, the extension does not crash.

The query param checking/signaling code looks like it does what it needs to.