Ensure tokens are added to both Trustor and Trustee indexes

Tokens are now added to both the Trustor and Trustee user-token-index
so that bulk token revocations (e.g. password change) of the trustee
will work as expected. This is a backport of the basic code that was
used in the Icehouse-vintage Dogpile Token KVS backend that resolves
this issue by merging the handling of memcache and KVS backends into
the same logic.

Change-Id: I3e19e4a8fc1e11cef6db51d364e80061e97befa7
Closes-Bug: #1260080