[subscriber] cache value

If value is not changed use one read previously

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
use is_subscribed_ to control subscription

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[server] better config monitor initialization

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
cosmetic changes

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[server] fix: start monitor before getattr too

It was started for root before readdir but also should be started before
getattr

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>