Bug 6794 - Deprecate eventbus, async-eventbus and fixed-threadpool

Those modules aren't used anymore, I couldn't find any reference of them,
expect some in netconf regarding the fixed-threadpool. But those
reference are coming from big xml test files that I suppose are pretty odl.

This deprecation notice is to warn potential downstream consumer that those
are now deprecated, and that those will be removed in next release, Nitrogen.

Change-Id: I8810b39a632c2d4c678c31bb46b8284aa33593f2
Signed-off-by: Alexis de Talhouët <adetalhouet@inocybe.com>