Add additional info to extraction end event

[bmitchinson]

Ticket Link #687

Description

• Updates readme with ingestion information
• The result of Processor.process now returns the amount of messages processed
  • The load_stage child process of Processor.process stores the messages processed in a dedicated cache
  • Test to affirm that the cache is updated after Processor.process
• data_extract_end event now includes ingestion start time and messages processed count

Reminders:

• If changing elixir code in an "app", did you update the relevant version
  in mix.exs?
• If altering an API endpoint, was the relevant postman collection updated?
  • If a new version of smart_city is being used (new fields on a struct), were the relevant postman collections updated?