side: explicit statedump request key

side: explicit statedump request key

Make the statedump request key explicit rather than hiding it as a
Thread-Local Storage variable within libside. It's type is still opaque
to the application (void *), but this allows schemes where the statedump
callbacks are not required to emit statedump events from the same
thread: it can pass the statedump_request_key to another thread
responsible for emitting those events.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>