Use Netty Timer in KeepAliveSalFacade

We are currently using ScheduledExecutorService to schedule both keepalive tasks and request timeouts. Most of the time we do not these timeouts to actually trigger, which means we end up scheduling/cancelling timeouts quite a bit.

In this scenario Netty's HashedWheelTimer offers superior performance, as task cancellation is O(1).

Switch to using Netty Timer to schedule timeouts.