Removing the mock nio transport and replacing its usage with the netty transport to make tests
with a more realistic transport implementation. This way improves the real world coverage for
the Netty transport, makes our tests more realistic and saves lots of code.
In particular, coverage on the rather complicated throttling/chunking in the netty message handler
is really ice to have.
The downside of this change is that we lose the slow transport thread warnings that the mock transport
outputs. This isn't a big deal these days in my opinion as we have slow logging in other places
now that makes up for this (we didn't when initially adding the slow logging) and that contains
far more detailed information on what exactly was slow.
Other than that, the mock transport does not come with any features we don't also have in the Netty
transport at this point.
0b66c14b7e