CRM_Core_BAO_Financial_Trxn::getFinancialTrxnIds() assumes only one financial_trxn linked to a contribution record

While working on fix for CRM-11668, realized that CRM_Core_BAO_Financial_Trxn::getFinancialTrxnIds() which is used by several classes returns single INTEGER values for $ids['entityFinancialTrxnId'] and $ids['financialTrxnId']

Given that most updates done to a contribution record will result in insertion of additional financial_trxn rows with entity_table = 'civicrm_contribution' - the query on line 132 will often return multiple rows. If we 'know' that we always want the first financial_trxn for the various classes which use this function - then we could just sort by ID ASC with LIMIT 1. Otherwise, please evaluate whether we should return an array of ID's - and update the classes which use this accordingly.