Related to #6734.
This first stage splits out the shared utilities in
`:instrumentation:netty:netty-common`. I'll follow it up by splitting
out `:instrumentation:netty:netty-4-common`,
`:instrumentation:netty:netty-4.1` in separate PRs. If there is
appetite, I can also split out library instrumentation for
`:instrumentation:netty:netty-4.0` and
`:instrumentation:netty:netty-3.8`, though I have no need for these.
jack-berg