Persistent delay buffer to correctly run slayer blocks one timestep at a time

[bamsumit]

Issue Number: 168

Objective of pull request:
Enable slayer blocks to run correctly when run one timestep at a time.

Pull request checklist

Your PR fulfills the following requirements:

• Issue created that explains the change and why it's needed
• Tests are part of the PR (for bug fixes / features)
• Docs reviewed and added / updated if needed (for bug fixes / features)
• PR conforms to Coding Conventions
• PR applys BSD 3-clause or LGPL2.1+ Licenses to all code files
• Lint (flakeheaven lint src/lava tests/) and (bandit -r src/lava/.) pass locally
• Build tests (pytest) passes locally

Pull request type

Please check your PR type:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation changes
• Other (please describe):

What is the current behavior?

• Slayer blocks don't run correctly when run one timestep at a time.

What is the new behavior?

• Slayer blocks run correctly when run one timestep at a time.

Does this introduce a breaking change?

• Yes
• No

Supplemental information