Revise `TcpListener.onMessage()` contract

[artembilan]

Looks like the boolean return from this method is ignored anywhere in the project.
Many places just always returns false.
Plus in some places this method called as like it is void.

The suggestion is to bite a bullet and change this contract to void.
Even if this is a breaking change, it should be OK for the current major version.

CC: @garyrussell

[garyrussell]

My recollection is that this was needed by the more exotic connection factories (cache, failover, etc.) but I have a recollection that the interceptor logic was changed at some later time to make it simpler.

If the result is now never tested by the framework then I agree that void makes sense.

There is a test interceptor that still returns true.

[artembilan]

Thank you, Gary, for confirmation!
Much appreciate!