Restructure message pool to increase maintainabillity and readability

[akaladarshi]

Summary

Right now message pool is not correctly structured:

• it has a lot of free and redundant methods
• Some methods accepts well over allowed limit of params
• For every small change we need to increase params accepted by these free functions

Completion Criteria

• Split the message pool (msg_pool.rs) file into multiple files, containing relevant peices
• Reduce the number of free methods
• Create common structures for the closely related fields
• Individual components should be testable
• Convert free functions into methods that belongs to message pool structure
• Wrap message pool in Arc and pass it to different modules and different process (head_change, republish_message)

Additional Links & Resources