Make linphone_core_interpret_url2 lowercase URI

Make case insesitive comparison of two objects of type ConferenceId

As address comparison is case insensitive as far as the parameters are
concerned (https://www.rfc-editor.org/rfc/rfc3261#section-19.1.4),
retrieve all conference infos from the database in order to properly
link a call log to a conference info

liblinphone!3058 (merged) https://linphone.atlassian.net/browse/SDK-164