BUG-633: allow overriding of channelFactory

It turns out netty does not let us override an already-set
channelFactory. Reorder customization to detect when such an override is
necessary.

Change-Id: I99de69b98e4034d4701313942d9ed2a09923375d
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-633: allow overriding of channelFactory"