public void close() {
        try {
            fileCache.close();
        } catch (Exception e) {
            log.log(Level.WARNING, "Exception when closing watcher", e);
        }
    }
Watches session state for a session (/config/v2/tenants/&lt;tenantName&gt;/sessions/&lt;n&gt;/sessionState in ZooKeeper)
The session must be in the session repo.

@author Vegard Havdal
@author hmusum
configserver/src/main/java/com/yahoo/vespa/config/server/session/SessionStateWatcher.java