[rts] Add parallelize() helper
[rts] Add parallelize() helper

Extract parallelization code into a separate helper function.
The next CL will add one more callsite.

Bug: 1112125
Change-Id: I1073271ea9187133362dbefb68a5c2d16835ef77
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2592654
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Gregory Guterman <8788ef0c8716b810f5f3059185bc1437e34caf28@google.com>
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

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