recordtester: Use random serf memeber for playback if enabled.

[hjpotter92]

Fixes #227

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Adds 3 new cli flags:

• --serf-rpc-addr: Used to connect to serf cluster and fetch list of current members
• --use-serf: Whether to really use serf cluster member for playback URL. Default is false.
• --random-serf-member: Uses a random selection from serf members list for playback. Otherwise uses the first member from the list.

[iameli]

This is a good (and mergeable) first step, but I think we'll want two more complex things here:

1. "Pull count" parameter so you can pull from multiple nodes; each one will need to trigger a separate playback
2. We'll want this to be able to geolocate so we're not unnecessarily testing pulls across the world. I guess this could be implemented with Serf tags to add the latitude and longitude parameters, and then a -max-distance parameter that limits how far the randomized Serf nodes are checked. (I guess we'll also need local -latitude and -longitude parameters to make that happen)

[hjpotter92]

1. "Pull count" parameter so you can pull from multiple nodes; each one will need to trigger a separate playback

added a TODO with unimplemented feature for this.

2. We'll want this to be able to geolocate so we're not unnecessarily testing pulls across the world. I guess this could be implemented with Serf tags to add the latitude and longitude parameters, and then a -max-distance parameter that limits how far the randomized Serf nodes are checked. (I guess we'll also need local -latitude and -longitude parameters to make that happen)

sounds like a plan. let me see if i can update serf tags.

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merging this PR at the current stage. will implement geolocation and pull-count once serf tags are updated.