We never do any file IO or other blocking work on the transport threads
so no tangible benefit can be derived from using more threads than CPUs
for IO.
There are however significant downsides to using more threads than necessary
with Netty in particular. Since we use the default setting for
`io.netty.allocator.useCacheForAllThreads` which is `true` we end up
using `16MB` of thread local buffer cache for each transport thread.
Meaning we potentially waste 2 * CPUs * 16MB of heap across both tcp
and http transports.

…read-counts