This patch modifies the Mapping class to prevent a mapping's
NotifyCallback from being called more than once if the mapping's state
hasn't changed (which can cause bugs, cf. the GitLab issue linked
below).

The patch also fixes a race condition in the reserveMapping function
where a mapping reserved by one thread could wrongly appear to still be
available from the perspective of a different thread.

https://git.jami.net/savoirfairelinux/jami-client-ios/-/issues/410

Change-Id: I3784d36ccbb2f278ecc1c2573ac6f36b125f58c1