Make each soundwave dashboard API request use a new connection
Make each soundwave dashboard API request use a new connection

This is a step towards making requests to the dashboard API in parallel.
App engine credentials are thread-safe, but connections are not. Because
connections are pretty lightweight to create, we can stash the
credentials instead of a connection and just use that credentials object
to create a new connection each time we make a request to the dashboard.
This makes the move to multithreading much easier.

Change-Id: Ic37bf5c1e1058d664a2bb33ec5f02791aa40c3be
Reviewed-on: https://chromium-review.googlesource.com/782100
Reviewed-by: rnephew <38d5ef163aefacaf06cee18d19b99b7f861ac4fd@chromium.org>
Commit-Queue: Charlie Andrews <532d7316e2689bea110898693dc3dc5671811b69@chromium.org>

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