At this point in riak's history disterl is our primary bottleneck.
Something about the way that it is framing messages is causing its
tcp connection to size its send and recv buffers much smaller than
is optimal.

This change adds code to automatically set dist port sizes on nodeup,
as well as code to manually set all disterl connections to specific
sizes, meant to be used from the console for testing and tuning.

Small refactorings:

* Store the send & recv buffer sizes in #state so that nodeup events
that arrive after a set_dist_buf_sizes() call will use the specified
sizes rather than OTP app environment var defaults.

* Create get_riak_env_vars/0 helper function and export it for debugging use
After refactoring further, it's only used once internally, so if YMMV,
I'm fine with inlining this function's code and removing the export.

* Add explicit buffer size args to set_port_buffers().

Variable renaming to match inet:setopts() option names

- remove redundant supervisor.

allow feature to be disabled