Add optional eventsocket Server to ndt-server

[stephen-soltesz]

This change adds an eventsocket.Server to the ndt-server for the ndt7 protocol.

With this change, the ndt-server could operate as a replacement for tcp-info's eventsocket for deployments that run with standard sidecar services (e.g. uuid-annotator, or others) but without needing tcp-info to be the event source. In this configuration, the ndt-server itself becomes the event source, limiting events to verified ndt7 measurements (rather than every TCP connection).

For this change to be possible, the ndt7 handler.Handler methods now use a pointer receiver so that the Handler.Events field can be reset after ndt7test.NewNDT7Server creates a server using the default eventsocket.NullServer. As well, we add a handler_integration_test.go to allow import of the existing ndt7test package, which sets up an ndt7 server for testing.

At the same time, I've removed use of the deprecated "io/ioutil" TempDir in favor of t.TempDir().

Default server behavior is unchanged. This change adds a new flag to the ndt-server, -tcpinfo.eventsocket string. We reuse the existing eventsocket flag name, just as the sidecars do, to preserve the symmetry between the serving flag and the client flag names. Since we now have multiple event servers, ideally the eventsocket package would be moved to a generic location such as m-lab/go/eventsocket and imported by tcp-info and ndt-server, using a more general flag name in all cases and sidecars, e.g. -server.eventsocket.

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This change is 

[cristinaleonr]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @stephen-soltesz)

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ndt7/handler/handler.go line 117 at r2 (raw file):

	h.Events.FlowCreated(result.StartTime, data.UUID, id)

	// Guarantee results are written even if test functions panics.

panics?

[stephen-soltesz]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @cristinaleonr)

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ndt7/handler/handler.go line 117 at r2 (raw file):

Previously, cristinaleonr (Cristina Leon) wrote…

panics?

Thank you. Fixed.