[resultdb] Use hex instead of base64
[resultdb] Use hex instead of base64

When generating an invocation ID, use hex encoding for the random suffix
instead of base64, because the latter has characters not allowed
in invocation IDs.

Change-Id: Iae79058bdf9c5436cb003aa1dbeb69f4729291b7
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2204873
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Reviewed-by: Scott Lee <1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org>

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