Revert "swarming: remove _EXECUTORS_PER_SHARD"
Revert "swarming: remove _EXECUTORS_PER_SHARD"

This reverts commit b36f18b86f73ca3215ef9be813c31a6e36250b90.

Reason for revert: https://crbug.com/1151408#c28

Original change's description:
> swarming: remove _EXECUTORS_PER_SHARD
>
> I think we don't need to have this item number limit.
>
> Bug: 1151408
> Change-Id: I7372bd0769ca9aa71c0251da12911e6c57a6a3a8
> Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2695249
> Commit-Queue: Junji Watanabe <jwata@google.com>
> Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
> Reviewed-by: Junji Watanabe <jwata@google.com>

Bug: 1151408
Change-Id: Ib3e0a52c1f12df55a32a966f0d2c58b02954434e
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2717124
Bot-Commit: Rubber Stamper <d516c4cc2192058ab8dae3d306acbdaf2ce6574f@appspot.gserviceaccount.com>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py