Darren/feat/add subscription cache

[darrenvechain]

Description

This PR leverages an LRU cache for beat, beat2 and block subscription endpoints. It generates the messages, marshals them and caches the bytes against thor block ID.

This improves CPU performance a lot when:

• There are a high amount of transactions, eg 90% gas utilization
• There are high number of beat/ beat2 websocket connections

How Has This Been Tested?

• Performance tested using https://github.com/darrenvechain/toolchain & a simple k6 script to create 10,000 websockets with an isolated node on testnet.
• Unit tests

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my 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
• New and existing E2E tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have not added any vulnerable dependencies to my code

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 72.64957% with 32 lines in your changes missing coverage. Please review.

Project coverage is 60.57%. Comparing base (a0e5a44) to head (8f752a7).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
api/subscriptions/message_cache.go	55.88%	10 Missing and 5 partials ⚠️
api/subscriptions/beat_reader.go	85.71%	3 Missing and 1 partial ⚠️
api/subscriptions/beat2_reader.go	88.00%	2 Missing and 1 partial ⚠️
api/subscriptions/block_reader.go	75.00%	2 Missing and 1 partial ⚠️
api/subscriptions/event_reader.go	57.14%	2 Missing and 1 partial ⚠️
api/subscriptions/transfer_reader.go	57.14%	2 Missing and 1 partial ⚠️
api/subscriptions/subscriptions.go	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #866      +/-   ##
==========================================
- Coverage   62.63%   60.57%   -2.06%     
==========================================
  Files         209      210       +1     
  Lines       18952    22498    +3546     
==========================================
+ Hits        11870    13628    +1758     
- Misses       5982     7760    +1778     
- Partials     1100     1110      +10     

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[darrenvechain]

TODO: Can we handle this more gracefully? Currently subscriptions.New nor api.New return errors, so this would mess up the flow a little bit