Command to dump the metadata cache again

This command allows the consumer to ask for the metadata cache to be
dumped entirely another time. This is used by the session rotation
feature to get a new copy of what was in the metadata cache without
regenerating it and re-sampling the offset from epoch.

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