Improve ReadTotalTimeout to start on reading first block

Andy Schoenenrequested to merge
andysoiron/start-http-read-total-timer-after-first-block into master

What does this MR do and why?

Initially, the timer for the ReadTotalTimeout started counting on the beginning of the request. For large uploads like LFS this could be problematic because the timeout can be exceeded even before the server had a chance to respond.

Related discussion: !66487 (comment 800938728)

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