Netty Replicator - improve the reconnection and keepalive mechanisms

Netty Replicator - improve the reconnection and keepalive mechanisms

In some cases during a network partition the disconnected
channel got closed with delay after a new channel was already created.
This started reconnection process which closed the new channel and
created yet another one.

Also improve the keepalive mechanism since some types of network
partitions left one side unaware of the issue.  It is important both
the Sink and the Source are notified about any connection issue as
soon as possible. PING-PONG messages are exchanged between the two
sides when no deltas are sent for a period of time

Configuration options keepalive-interval-seconds and
max-missed-keepalives added to both configurations.

Change-Id: Iebde72963bddb748ab97617d07cfc77cd8614da4
Signed-off-by: Tibor Král <tibor.kral@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>