swarming: trap RPC deadline exception and create a unique signature
swarming: trap RPC deadline exception and create a unique signature

In this case, skip the whole polling RPC and return nothing to the bot. This
acts as a back-pressure to reduce contention on Cloud DB indexes, and returns an
HTTP 200 instead of an HTTP 500, yet still logs an error to keep visibility.

R=9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org

Bug:
Change-Id: I872d70ea8210fc9d9cec1cea8a0f1d3bbbd7030b
Reviewed-on: https://chromium-review.googlesource.com/765265
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

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