(backport) Move linphone_core_iterate to core.cpp
https://linphone.atlassian.net/browse/SDK-782 liblinphone: liblinphone!3716 (merged) Original master mr: !5313 (merged)
https://linphone.atlassian.net/browse/SDK-782 liblinphone: liblinphone!3716 (merged) Original master mr: !5313 (merged)