Set default metadata backlog to 4096

We already optimize the metadata agent by running it in multi-workers,
we need by default to set the metadata backlog value.
Inspired from 19:34: https://www.youtube.com/watch?v=AF9r_VQrcJ0
and https://review.openstack.org/#/c/95372

Change-Id: I68f3414ef56290ca27d11f8cf26febc100741d46
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>