[tqtesting] better identifications of tasks.
[tqtesting] better identifications of tasks.

Previously, I used counter of task executions.
However, this doesn't provide for easy observation of retries.
So, use task name, which is almost always unique and a number
of its executions so far as a value. In logs, it looks like this:

  ... components.go:224] bla bla ... {"TQ-4577033f":0}
  ...
  ... components.go:224] bla bla ... {"TQ-4577033f":1}

R=qyearsley, yiwzhang

Change-Id: I087486347b64b1aaaa3d06e550ac3db65ff2dfd4
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2928773
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Quinten Yearsley <747cf52e0cef2e3e9371802a2d344b6c87126a4b@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go