scheduler: pure refactoring of task/gitiles refs processing.
scheduler: pure refactoring of task/gitiles refs processing.

 * more concise parallel loading of remote and saved refs.
 * combine refs related state into refsState struct
 * shorten long LaunchTask function for readability

R=9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org

Change-Id: Icb804eed225fe372791e0db71816e3bfe11ba3d6
Reviewed-on: https://chromium-review.googlesource.com/1053383
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>

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