swarming: increase a deadline for datastore access
swarming: increase a deadline for datastore access

Due to 15s deadline, /handshake and /poll failed often with timeout and
that caused many retry of http request from bot and caused many datastore access.
I think increasing this deadline will mitigate this issue.

This deadline was introduced in
https://chromium.googlesource.com/infra/luci/luci-py/+/83275bf1f61bac16b51781ead87ce46ea238d8da%5E%21/#F2
but that seems not sufficient for chromeos workload.

Bug: 1020011
Change-Id: I7598d7b27adaacc273a99f271ce1a85b962164b6
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1890466
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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