Bug-2080: BGP statistics.

-per BGP peer statistics, related with bgp-peer config module

-peer and speaker preferences
	-proposed holdtimer-value, BGP ID, AS number, Ip address, port, 4-byte AS capa., advertized routing tables
-session's holdtimer value and keep-alive
-received/sent BGP messages statistics (last timestamp, count)
	-all msgs, keep-alive msgs, update msgs, notification msgs (error code and subcode)
-number of time the session was established
-routes count per routing table
-op. reset statistics
-op. reset session

-statistics reachable via Netconf, Restconf - as operational data and JMX (JConsole, VisulaVM, Jolokia) - as Runtime Bean.

Change-Id: I797535744176eea61d538dfa00d23cc6bfdd3da2
Signed-off-by: Milos Fabian <milfabia@cisco.com>