Optimize indexing process for field processors

Purpose/Overview: 

Currently, all of field processors use the JsonConverter to convert map value to the field of the Instance model. It would be faster and more convenient to convert map value to Instance once, and use it as source for field extraction.

Requirements/Scope:

1. JsonConverter should be called once to convert map value to the Instance for field processors

Acceptance criteria:

• New implementation covered by unit tests
• Integration and unit tests passed through