Upgrade to netty 4.0.21

Open Tasks:

- Decide what to do with shared-event-executor (src/riemann/transport.clj).

The shared-event-executor is replacing the old thread pool (because it was
deprecated).
But there might be a better implementation available which gives you a similar
degree of control.

This should probably be addressed before the next release of riemann.

The current value is set to 100 (100 available threads) using the
DefaultEventExecutorGroup.

- Test cases might be misbehaving.

riemann.graphite-test seems to be displaying an error which might be
a programming error. An exception that is not being caught by implemention the
correct pipeline event handler.

I'm not sure how to address this.
Added Datadog plugin
Remove unused leiningen build parser task
Indexes correctly persist through reloads.

Prior to this commit, the old index would be preserved correctly by the
core on reload, but the new set of streams would have a reference to a
*different* index. Just like config/service, config/index needs to check
for equivalency and return the current core's index if core/merge will
preserve it.

I believe this also fixes #473
Update service-equiv tests for indices

wrapped/unwrapped index are not equivalent.
Merge pull request #451 from pradeepchhetri/datadog

Added Datadog plugin
Merge pull request #432 from udoprog/netty-4

Upgrade to netty 4.0.21
Port OpenTSDB server to Netty 4

Also add round-trip tests for the OpenTSDB server. Noticed that wow,
this thing doesn't round-trip well. Not sure if this is intentional;
I'll defer to folks who understand the opentsdb data model better.
Graphite server roundtrip test