Data can now be stored on thread-local storage of the worker. By acquiring
a unique handle from the worker, a module can store a thread-local
value.
This functionality will be used to store configurations that are sometimes
updated at runtime but are largely read-only. By avoiding shared data
altogether, performance is not affected. The only synchronization that is
done is on update.
Also added a helper functions for broadcasting tasks on all routing
workers. With the old mxs_rworker_broadcast_message function, if a
function call was broadcasted it was always queued for execution. The
mxs_rworker_broadcast will immediately execute the task on the local
worker and queue it for execution of other routing workers.
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