Add sent-event-records counter
Added updateReceiver() method to MdsalRestconfStreamRegistry to update
per-receiver counters in the operational datastore.
Added ReceiverHolder to encapsulate receiver identification
(subscriptionId, receiverName) and provide access to local counter
values. Added ChannelSenderSubscription class to handle HTTP/1
event streaming and update per-receiver counters via the Registry.
JIRA: NETCONF-1449
Change-Id: Ifa9f101d6d27b13ef74bf5e03084bddbb83af66b
Signed-off-by: Yaroslav Lastivka <yaroslav.lastivka@pantheon.tech>