Deactivating an account currently does not purge active SSH connections
(E.g.: long-running stream-events) and does not purge the web session
cache. So deactivating a user for real requires to also manually purge
these connections and caches, which is unnecessarily tedious.
Implementing such code directly in SetInactiveFlag is not straight
forward as com.google.server.account does not have access to the sshd
package, and naively adding it to the build introduces a circular
dependency. So we add an extension point for account deactivation.
Thereby, core components and plugins can self-cater cache purging and
other house-holding tasks upon user deactivation without messing with
the build.
Change-Id: I1dfa1779acfe56e5c7f1666b1e83e83f954083e1
db930bd5c7