Create RequestManager and SubscriptionManager classes

[nazarhussain]

Description

Create the implementation of RequestManager and SubscriptionManager.

Fixes #4429, #4430

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• I have selected the correct base branch.
• 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.
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run dtslint with success and extended the tests and types if necessary.
• I ran npm run test:unit with success.
• I ran npm run test:cov and my test cases cover all the lines and branches of the added code.
• I ran npm run build and tested dist/web3.min.js in a browser.
• I have tested my code on the live network.
• I have checked the Deploy Preview and it looks correct.
• I have updated the CHANGELOG.md file in the root folder.

[lgtm-com]

This pull request introduces 2 alerts when merging aef3292 into 4428570 - view on LGTM.com

new alerts:

• 2 for Useless conditional

[lgtm-com]

This pull request introduces 2 alerts when merging 6ff4afa into 09fc675 - view on LGTM.com

new alerts:

• 2 for Useless conditional

[lgtm-com]

This pull request introduces 2 alerts when merging ff3bf0f into 09fc675 - view on LGTM.com

new alerts:

• 2 for Useless conditional