Mark kvs backends as deprecated, for removal in Kilo.

Although kvs.Base was marked from removal in Juno, the backends
themselves were not explicitly marked. This patch does this,
as well as extendes kvs.Base also to Kilo so that they can all
be removed together.

Partially implements: bp deprecated-as-of-juno
Change-Id: Ia2e7c95074c4b6b9743c6086842105a5579fc0c9
Merge "Mark kvs backends as deprecated, for removal in Kilo."