GUILD-555: Don't resend messages when splitting up batches

[DeeChau]

Pull Request Template

Description

• Do not resend already sent messages when splitting up batches in db_producer
  • Uses a counter to keep track of the sent messages to kafka
  • When accessing the batch, index directly into the Array[Hash] to the current location of the batch
• Adds a document called INTEGRATION_TESTS.md for running integration tests such as db_poller and db_producer/db backend
• Updated integration unit tests for db_producer

Fixes #24

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• This change requires a documentation update

How Has This Been Tested?

• Integration Unit Tests

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules