certificateModel: Add a new mutex for the cache insertion

It isn't the same operation as the already protected sections.
The new mutex is specific to m_hCertificates insertions.

Change-Id: Ib178fa216a7dd082aba18b5131b87271b1d0a4e8
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>