[swarming] add 'bot_missing' event type
[swarming] add 'bot_missing' event type

Add a new bot event 'bot_missing'. This should be deployed before deploying a new code use it.
See also:
https://crrev.com/c/1916984/5/appengine/swarming/server/bot_management.py#366

Bug: 916578
Change-Id: Ib1ef2f3e2379173f1f47e4ca7a8d06df975a4ef9
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1916993
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

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