Feature/674 servicepath in notifications

### Description
HTTP header 'Service-Path' in notifications.
Solves issue #674

I'd add two more test:

* One test with a simple case: just one entity belonging to "/entity" and an ONCHANGE subscription to it.
* One test checking hierarchical scopes: one entity belonging to /entity (E1), other entity belonging to /entity/path (E2). Subscription with "/entity/path" returns only E2. Subscription without servicepath returns both E1 and E2. Subscriptions to "/entity" returns only E1. Subscription to "/entity/#" returns both E1 and E2.