swarming: disable TaskToRun caching
swarming: disable TaskToRun caching

Disable caching TaskToRun for both in-process and memcache. There's no point
since the entity is small and it's always written and read in a transaction.

This will help (a bit):
- reduce memory usage for handlers passing over a large number of entities
- reduce overhead RPCs for memcache as they are not useful anyway

R=9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org
Bug:
Change-Id: I37e1f114a8c35991acf6967ab4c641539d762436
Reviewed-on: https://chromium-review.googlesource.com/764655
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