Add usage to Partner API endpoint for IAAs

[orenyk]

Part of LG-4236

• Add an SpReturnLogScan query object that does a table scan of the
  sp_return_logs table and yields each record
• Add an IaaUsage service object that is instantiated with an IaaOrder
  and implements a #count method to take an sp_return_log record and
  update an authentications array and ial2_users set if appropriate
• Add a UsageSummarizer service object that users the SpReturnLogScan
  and IaaUsage to reduce all of the sp_return_log records into a
  collection of IaaUsages
• Add ial2_auths and authentications attributes to Iaa model and
  serializer
• Add #in_pop? method to IaaOrder
• Modify the PartnerApiReport to run the usage summary and update the
  list of IAAs with the relevant usage data
• Add model spec for SpReturnLog