[impl/memory] Use raw (unpadded) base64 encoding for cursors.
[impl/memory] Use raw (unpadded) base64 encoding for cursors.

This is what real datastore library does. This is important for rare cases when
callers want to decode the cursor back to bytes and put it into some other
high-lever cursor object.

R=a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org, iannucci@chromium.org, nodir@chromium.org, jchinlee@chromium.org

Change-Id: If839b142df573ebdfa357672862a71ea1c5ad806
Reviewed-on: https://chromium-review.googlesource.com/1012983
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go