erasure-code: assert the PluginRegistry lock is held when it must

erasure-code: assert the PluginRegistry lock is held when it must

Add lock to the preload method and assert that it is held by methods
requiring it. Although preload is called at bootstrap and does not
require the lock, adding it does not hurt and makes the lock policy
clearer to understand.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>