Replace per-request j.u.Timer with a Netty Timer

Replace per-request j.u.Timer with a Netty Timer

We are currently spawning a full java.util.Timer for every request we are
sending out. That is rather wasteful in terms of resources: each such
timer costs us a thread.

Rather than doing that, use Netty's HashedWheelTimer, which can easily
cope with a large number of timeouts.

JIRA: BGPCEP-1006
Change-Id: I4f3b6bf6a435170c8a054aa0e012418c60b17e96
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>