Switches from implementing a stateful registry to a registry that sends
all acitivity to a spectatord sidecar. The build only supports bazel
now, and we add a dependency on abseil and asio (removing the
curl/cares/rapidjson/boringssl, etc. dependencies)

Currently the travis build is failing but not giving a lot of detail
about what is causing an exception.

The latest build shows the exception happening because we didn't join a
thread which points to something throwing in one of the test servers
(udp/local)

Diagnosing the joinable thread throwing an exception during the
destructor of the test server

We were missing a virtual destructor for Publisher causing the
TestPublisher instance in our tests to not be destructed properly (the
dtor for Publisher was invoked through the pointer, and not
TestPublisher)