MINOR: ssl: Factorize ckch instance rebuild process

MINOR: ssl: Factorize ckch instance rebuild process

The ckch instances for a given ckch_store have to be rebuilt when a
certificate is updated during runtime (via cli or lua). The code was
duplicated in lua so factorizing the actual loop avoids future errors
if the code changes. The new 'ckch_store_rebuild_instances' will have a
dedicated 0 return code if it needs to be called again (because of the
yielding logic since ckch instance rebuild might take some time).