chore: Refactor older relay REST API to use new subscriptions

[chaitanyaprem]

Background

The existing relay REST and RPC API's use a local message cache to read from subcription and store messages. Since the subscription has been updated to cache messages, this additional cache is not required anymore and can be removed.

Details

• Modify older relay REST API to use cache from subscription chore: update relay REST and RPC API's and fix unit tests #866
• Modify relay RPC API to use cache from subscription Chore/relay rpc api update #867
• Add unit tests for autosharding REST API chore: add unit tests for autosharding relay REST API #868

Acceptance criteria

A list of tasks that need to be done for the issue to be considered resolved.