sse fix work area

trying to reproduce http://tracker.ceph.com/issues/7826

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: allow loading a plugin from factory()

The Mutex scope is restricted to only protect the load() method and not
the factory() method. This allows a plugin to load another plugin from
within the factory() method. This is convenient for the pyramid plugin
where each layer can specify a different plugin.

Reviewed-By: Christophe Courtaut <christophe.courtaut@gmail.com>
Signed-off-by: Loic Dachary <loic@dachary.org>
work

Signed-off-by: Loic Dachary <loic@dachary.org>