Bug 6442: Keepalive not send by BGP ODL

Bug 6442: Keepalive not send by BGP ODL

When ODL BGP is reading a bigger amount of data using
native epoll transport, scheduled KA hndler task is getting executed.
As a result, remote peer's holdtimer expires -> connection is dropped.

Switch the Epoll mode to "level-triggered", as we want to use MAX_MESSAGES_PER_READ
channel option (round-robin processing).

Ref.: https://netty.io/4.0/api/io/netty/channel/epoll/EpollChannelConfig.html#setEpollMode(io.netty.channel.epoll.EpollMode)

Change-Id: If4a6ac3e3fc9d006cd6ba54c1606731add8bfa2c
Signed-off-by: Milos Fabian <milfabia@cisco.com>